manyi/venv/Lib/site-packages/cssselect/xpath.py

# -*- coding: utf-8 -*-
"""
    cssselect.xpath
    ===============

    Translation of parsed CSS selectors to XPath expressions.


    :copyright: (c) 2007-2012 Ian Bicking and contributors.
                See AUTHORS for more details.
    :license: BSD, see LICENSE for more details.

"""

import sys
import re

from cssselect.parser import parse, parse_series, SelectorError


if sys.version_info[0] < 3:
    _basestring = basestring
    _unicode = unicode
else:
    _basestring = str
    _unicode = str


def _unicode_safe_getattr(obj, name, default=None):
    # getattr() with a non-ASCII name fails on Python 2.x
    name = name.encode('ascii', 'replace').decode('ascii')
    return getattr(obj, name, default)


class ExpressionError(SelectorError, RuntimeError):
    """Unknown or unsupported selector (eg. pseudo-class)."""


#### XPath Helpers

class XPathExpr(object):

    def __init__(self, path='', element='*', condition='', star_prefix=False):
        self.path = path
        self.element = element
        self.condition = condition

    def __str__(self):
        path =  _unicode(self.path) + _unicode(self.element)
        if self.condition:
            path += '[%s]' % self.condition
        return path

    def __repr__(self):
        return '%s[%s]' % (self.__class__.__name__, self)

    def add_condition(self, condition):
        if self.condition:
            self.condition = '%s and (%s)' % (self.condition, condition)
        else:
            self.condition = condition
        return self

    def add_name_test(self):
        if self.element == '*':
            # We weren't doing a test anyway
            return
        self.add_condition(
            "name() = %s" % GenericTranslator.xpath_literal(self.element))
        self.element = '*'

    def add_star_prefix(self):
        """
        Append '*/' to the path to keep the context constrained
        to a single parent.
        """
        self.path += '*/'

    def join(self, combiner, other):
        path = _unicode(self) + combiner
        # Any "star prefix" is redundant when joining.
        if other.path != '*/':
            path += other.path
        self.path = path
        self.element = other.element
        self.condition = other.condition
        return self


split_at_single_quotes = re.compile("('+)").split

# The spec is actually more permissive than that, but don’t bother.
# This is just for the fast path.
# http://www.w3.org/TR/REC-xml/#NT-NameStartChar
is_safe_name = re.compile('^[a-zA-Z_][a-zA-Z0-9_.-]*$').match

# Test that the string is not empty and does not contain whitespace
is_non_whitespace = re.compile(r'^[^ \t\r\n\f]+$').match


#### Translation

class GenericTranslator(object):
    """
    Translator for "generic" XML documents.

    Everything is case-sensitive, no assumption is made on the meaning
    of element names and attribute names.

    """

    ####
    ####  HERE BE DRAGONS
    ####
    ####  You are welcome to hook into this to change some behavior,
    ####  but do so at your own risks.
    ####  Until is has recieved a lot more work and review,
    ####  I reserve the right to change this API in backward-incompatible ways
    ####  with any minor version of cssselect.
    ####  See https://github.com/scrapy/cssselect/pull/22
    ####  -- Simon Sapin.
    ####

    combinator_mapping = {
        ' ': 'descendant',
        '>': 'child',
        '+': 'direct_adjacent',
        '~': 'indirect_adjacent',
    }

    attribute_operator_mapping = {
       'exists': 'exists',
        '=': 'equals',
        '~=': 'includes',
        '|=': 'dashmatch',
        '^=': 'prefixmatch',
        '$=': 'suffixmatch',
        '*=': 'substringmatch',
        '!=': 'different',  # XXX Not in Level 3 but meh
    }

    #: The attribute used for ID selectors depends on the document language:
    #: http://www.w3.org/TR/selectors/#id-selectors
    id_attribute = 'id'

    #: The attribute used for ``:lang()`` depends on the document language:
    #: http://www.w3.org/TR/selectors/#lang-pseudo
    lang_attribute = 'xml:lang'

    #: The case sensitivity of document language element names,
    #: attribute names, and attribute values in selectors depends
    #: on the document language.
    #: http://www.w3.org/TR/selectors/#casesens
    #:
    #: When a document language defines one of these as case-insensitive,
    #: cssselect assumes that the document parser makes the parsed values
    #: lower-case. Making the selector lower-case too makes the comparaison
    #: case-insensitive.
    #:
    #: In HTML, element names and attributes names (but not attribute values)
    #: are case-insensitive. All of lxml.html, html5lib, BeautifulSoup4
    #: and HTMLParser make them lower-case in their parse result, so
    #: the assumption holds.
    lower_case_element_names = False
    lower_case_attribute_names = False
    lower_case_attribute_values = False

    # class used to represent and xpath expression
    xpathexpr_cls = XPathExpr

    def css_to_xpath(self, css, prefix='descendant-or-self::'):
        """Translate a *group of selectors* to XPath.

        Pseudo-elements are not supported here since XPath only knows
        about "real" elements.

        :param css:
            A *group of selectors* as an Unicode string.
        :param prefix:
            This string is prepended to the XPath expression for each selector.
            The default makes selectors scoped to the context node’s subtree.
        :raises:
            :class:`SelectorSyntaxError` on invalid selectors,
            :class:`ExpressionError` on unknown/unsupported selectors,
            including pseudo-elements.
        :returns:
            The equivalent XPath 1.0 expression as an Unicode string.

        """
        return ' | '.join(self.selector_to_xpath(selector, prefix,
                                                 translate_pseudo_elements=True)
                          for selector in parse(css))

    def selector_to_xpath(self, selector, prefix='descendant-or-self::',
                          translate_pseudo_elements=False):
        """Translate a parsed selector to XPath.


        :param selector:
            A parsed :class:`Selector` object.
        :param prefix:
            This string is prepended to the resulting XPath expression.
            The default makes selectors scoped to the context node’s subtree.
        :param translate_pseudo_elements:
            Unless this is set to ``True`` (as :meth:`css_to_xpath` does),
            the :attr:`~Selector.pseudo_element` attribute of the selector
            is ignored.
            It is the caller's responsibility to reject selectors
            with pseudo-elements, or to account for them somehow.
        :raises:
            :class:`ExpressionError` on unknown/unsupported selectors.
        :returns:
            The equivalent XPath 1.0 expression as an Unicode string.

        """
        tree = getattr(selector, 'parsed_tree', None)
        if not tree:
            raise TypeError('Expected a parsed selector, got %r' % (selector,))
        xpath = self.xpath(tree)
        assert isinstance(xpath, self.xpathexpr_cls)  # help debug a missing 'return'
        if translate_pseudo_elements and selector.pseudo_element:
            xpath = self.xpath_pseudo_element(xpath, selector.pseudo_element)
        return (prefix or '') + _unicode(xpath)

    def xpath_pseudo_element(self, xpath, pseudo_element):
        """Translate a pseudo-element.

        Defaults to not supporting pseudo-elements at all,
        but can be overridden by sub-classes.

        """
        raise ExpressionError('Pseudo-elements are not supported.')

    @staticmethod
    def xpath_literal(s):
        s = _unicode(s)
        if "'" not in s:
            s = "'%s'" % s
        elif '"' not in s:
            s = '"%s"' % s
        else:
            s = "concat(%s)" % ','.join([
                (("'" in part) and '"%s"' or "'%s'") % part
                for part in split_at_single_quotes(s) if part
                ])
        return s

    def xpath(self, parsed_selector):
        """Translate any parsed selector object."""
        type_name = type(parsed_selector).__name__
        method = getattr(self, 'xpath_%s' % type_name.lower(), None)
        if method is None:
            raise ExpressionError('%s is not supported.' %  type_name)
        return method(parsed_selector)


    # Dispatched by parsed object type

    def xpath_combinedselector(self, combined):
        """Translate a combined selector."""
        combinator = self.combinator_mapping[combined.combinator]
        method = getattr(self, 'xpath_%s_combinator' % combinator)
        return method(self.xpath(combined.selector),
                      self.xpath(combined.subselector))

    def xpath_negation(self, negation):
        xpath = self.xpath(negation.selector)
        sub_xpath = self.xpath(negation.subselector)
        sub_xpath.add_name_test()
        if sub_xpath.condition:
            return xpath.add_condition('not(%s)' % sub_xpath.condition)
        else:
            return xpath.add_condition('0')

    def xpath_function(self, function):
        """Translate a functional pseudo-class."""
        method = 'xpath_%s_function' % function.name.replace('-', '_')
        method = _unicode_safe_getattr(self, method, None)
        if not method:
            raise ExpressionError(
                "The pseudo-class :%s() is unknown" % function.name)
        return method(self.xpath(function.selector), function)

    def xpath_pseudo(self, pseudo):
        """Translate a pseudo-class."""
        method = 'xpath_%s_pseudo' % pseudo.ident.replace('-', '_')
        method = _unicode_safe_getattr(self, method, None)
        if not method:
            # TODO: better error message for pseudo-elements?
            raise ExpressionError(
                "The pseudo-class :%s is unknown" % pseudo.ident)
        return method(self.xpath(pseudo.selector))


    def xpath_attrib(self, selector):
        """Translate an attribute selector."""
        operator = self.attribute_operator_mapping[selector.operator]
        method = getattr(self, 'xpath_attrib_%s' % operator)
        if self.lower_case_attribute_names:
            name = selector.attrib.lower()
        else:
            name = selector.attrib
        safe = is_safe_name(name)
        if selector.namespace:
            name = '%s:%s' % (selector.namespace, name)
            safe = safe and is_safe_name(selector.namespace)
        if safe:
            attrib = '@' + name
        else:
            attrib = 'attribute::*[name() = %s]' % self.xpath_literal(name)
        if self.lower_case_attribute_values:
            value = selector.value.lower()
        else:
            value = selector.value
        return method(self.xpath(selector.selector), attrib, value)

    def xpath_class(self, class_selector):
        """Translate a class selector."""
        # .foo is defined as [class~=foo] in the spec.
        xpath = self.xpath(class_selector.selector)
        return self.xpath_attrib_includes(
            xpath, '@class', class_selector.class_name)

    def xpath_hash(self, id_selector):
        """Translate an ID selector."""
        xpath = self.xpath(id_selector.selector)
        return self.xpath_attrib_equals(xpath, '@id', id_selector.id)

    def xpath_element(self, selector):
        """Translate a type or universal selector."""
        element = selector.element
        if not element:
            element = '*'
            safe = True
        else:
            safe = is_safe_name(element)
            if self.lower_case_element_names:
                element = element.lower()
        if selector.namespace:
            # Namespace prefixes are case-sensitive.
            # http://www.w3.org/TR/css3-namespace/#prefixes
            element = '%s:%s' % (selector.namespace, element)
            safe = safe and is_safe_name(selector.namespace)
        xpath = self.xpathexpr_cls(element=element)
        if not safe:
            xpath.add_name_test()
        return xpath


    # CombinedSelector: dispatch by combinator

    def xpath_descendant_combinator(self, left, right):
        """right is a child, grand-child or further descendant of left"""
        return left.join('/descendant-or-self::*/', right)

    def xpath_child_combinator(self, left, right):
        """right is an immediate child of left"""
        return left.join('/', right)

    def xpath_direct_adjacent_combinator(self, left, right):
        """right is a sibling immediately after left"""
        xpath = left.join('/following-sibling::', right)
        xpath.add_name_test()
        return xpath.add_condition('position() = 1')

    def xpath_indirect_adjacent_combinator(self, left, right):
        """right is a sibling after left, immediately or not"""
        return left.join('/following-sibling::', right)


    # Function: dispatch by function/pseudo-class name

    def xpath_nth_child_function(self, xpath, function, last=False,
                                 add_name_test=True):
        try:
            a, b = parse_series(function.arguments)
        except ValueError:
            raise ExpressionError("Invalid series: '%r'" % function.arguments)

        # From https://www.w3.org/TR/css3-selectors/#structural-pseudos:
        #
        # :nth-child(an+b)
        #       an+b-1 siblings before
        #
        # :nth-last-child(an+b)
        #       an+b-1 siblings after
        #
        # :nth-of-type(an+b)
        #       an+b-1 siblings with the same expanded element name before
        #
        # :nth-last-of-type(an+b)
        #       an+b-1 siblings with the same expanded element name after
        #
        # So,
        # for :nth-child and :nth-of-type
        #
        #    count(preceding-sibling::<nodetest>) = an+b-1
        #
        # for :nth-last-child and :nth-last-of-type
        #
        #    count(following-sibling::<nodetest>) = an+b-1
        #
        # therefore,
        #    count(...) - (b-1) ≡ 0 (mod a)
        #
        # if a == 0:
        # ~~~~~~~~~~
        #    count(...) = b-1
        #
        # if a < 0:
        # ~~~~~~~~~
        #    count(...) - b +1 <= 0
        # -> count(...) <= b-1
        #
        # if a > 0:
        # ~~~~~~~~~
        #    count(...) - b +1 >= 0
        # -> count(...) >= b-1

        # work with b-1 instead
        b_min_1 = b - 1

        # early-exit condition 1:
        # ~~~~~~~~~~~~~~~~~~~~~~~
        # for a == 1, nth-*(an+b) means n+b-1 siblings before/after,
        # and since n ∈ {0, 1, 2, ...}, if b-1<=0,
        # there is always an "n" matching any number of siblings (maybe none)
        if a == 1 and b_min_1 <=0:
            return xpath

        # early-exit condition 2:
        # ~~~~~~~~~~~~~~~~~~~~~~~
        # an+b-1 siblings with a<0 and (b-1)<0 is not possible
        if a < 0 and b_min_1 < 0:
            return xpath.add_condition('0')

        # `add_name_test` boolean is inverted and somewhat counter-intuitive:
        #
        # nth_of_type() calls nth_child(add_name_test=False)
        if add_name_test:
            nodetest = '*'
        else:
            nodetest  = '%s' % xpath.element

        # count siblings before or after the element
        if not last:
            siblings_count = 'count(preceding-sibling::%s)' % nodetest
        else:
            siblings_count = 'count(following-sibling::%s)' % nodetest

        # special case of fixed position: nth-*(0n+b)
        # if a == 0:
        # ~~~~~~~~~~
        #    count(***-sibling::***) = b-1
        if a == 0:
            return xpath.add_condition('%s = %s' % (siblings_count, b_min_1))

        expr = []

        if a > 0:
            # siblings count, an+b-1, is always >= 0,
            # so if a>0, and (b-1)<=0, an "n" exists to satisfy this,
            # therefore, the predicate is only interesting if (b-1)>0
            if b_min_1 > 0:
                expr.append('%s >= %s' % (siblings_count, b_min_1))
        else:
            # if a<0, and (b-1)<0, no "n" satisfies this,
            # this is tested above as an early exist condition
            # otherwise,
            expr.append('%s <= %s' % (siblings_count, b_min_1))

        # operations modulo 1 or -1 are simpler, one only needs to verify:
        #
        # - either:
        # count(***-sibling::***) - (b-1) = n = 0, 1, 2, 3, etc.,
        #   i.e. count(***-sibling::***) >= (b-1)
        #
        # - or:
        # count(***-sibling::***) - (b-1) = -n = 0, -1, -2, -3, etc.,
        #   i.e. count(***-sibling::***) <= (b-1)
        # we we just did above.
        #
        if abs(a) != 1:
            # count(***-sibling::***) - (b-1) ≡ 0 (mod a)
            left = siblings_count

            # apply "modulo a" on 2nd term, -(b-1),
            # to simplify things like "(... +6) % -3",
            # and also make it positive with |a|
            b_neg = (-b_min_1) % abs(a)

            if b_neg != 0:
                b_neg = '+%s' % (b_neg)
                left = '(%s %s)' % (left, b_neg)

            expr.append('%s mod %s = 0' % (left, a))

        xpath.add_condition(' and '.join(expr))
        return xpath

    def xpath_nth_last_child_function(self, xpath, function):
        return self.xpath_nth_child_function(xpath, function, last=True)

    def xpath_nth_of_type_function(self, xpath, function):
        if xpath.element == '*':
            raise ExpressionError(
                "*:nth-of-type() is not implemented")
        return self.xpath_nth_child_function(xpath, function,
                                             add_name_test=False)

    def xpath_nth_last_of_type_function(self, xpath, function):
        if xpath.element == '*':
            raise ExpressionError(
                "*:nth-of-type() is not implemented")
        return self.xpath_nth_child_function(xpath, function, last=True,
                                             add_name_test=False)

    def xpath_contains_function(self, xpath, function):
        # Defined there, removed in later drafts:
        # http://www.w3.org/TR/2001/CR-css3-selectors-20011113/#content-selectors
        if function.argument_types() not in (['STRING'], ['IDENT']):
            raise ExpressionError(
                "Expected a single string or ident for :contains(), got %r"
                % function.arguments)
        value = function.arguments[0].value
        return xpath.add_condition(
            'contains(., %s)' % self.xpath_literal(value))

    def xpath_lang_function(self, xpath, function):
        if function.argument_types() not in (['STRING'], ['IDENT']):
            raise ExpressionError(
                "Expected a single string or ident for :lang(), got %r"
                % function.arguments)
        value = function.arguments[0].value
        return xpath.add_condition(
            "lang(%s)" % (self.xpath_literal(value)))


    # Pseudo: dispatch by pseudo-class name

    def xpath_root_pseudo(self, xpath):
        return xpath.add_condition("not(parent::*)")

    def xpath_first_child_pseudo(self, xpath):
        return xpath.add_condition('count(preceding-sibling::*) = 0')

    def xpath_last_child_pseudo(self, xpath):
        return xpath.add_condition('count(following-sibling::*) = 0')

    def xpath_first_of_type_pseudo(self, xpath):
        if xpath.element == '*':
            raise ExpressionError(
                "*:first-of-type is not implemented")
        return xpath.add_condition('count(preceding-sibling::%s) = 0' % xpath.element)

    def xpath_last_of_type_pseudo(self, xpath):
        if xpath.element == '*':
            raise ExpressionError(
                "*:last-of-type is not implemented")
        return xpath.add_condition('count(following-sibling::%s) = 0' % xpath.element)

    def xpath_only_child_pseudo(self, xpath):
        return xpath.add_condition('count(parent::*/child::*) = 1')

    def xpath_only_of_type_pseudo(self, xpath):
        if xpath.element == '*':
            raise ExpressionError(
                "*:only-of-type is not implemented")
        return xpath.add_condition('count(parent::*/child::%s) = 1' % xpath.element)

    def xpath_empty_pseudo(self, xpath):
        return xpath.add_condition("not(*) and not(string-length())")

    def pseudo_never_matches(self, xpath):
        """Common implementation for pseudo-classes that never match."""
        return xpath.add_condition("0")

    xpath_link_pseudo = pseudo_never_matches
    xpath_visited_pseudo = pseudo_never_matches
    xpath_hover_pseudo = pseudo_never_matches
    xpath_active_pseudo = pseudo_never_matches
    xpath_focus_pseudo = pseudo_never_matches
    xpath_target_pseudo = pseudo_never_matches
    xpath_enabled_pseudo = pseudo_never_matches
    xpath_disabled_pseudo = pseudo_never_matches
    xpath_checked_pseudo = pseudo_never_matches

    # Attrib: dispatch by attribute operator

    def xpath_attrib_exists(self, xpath, name, value):
        assert not value
        xpath.add_condition(name)
        return xpath

    def xpath_attrib_equals(self, xpath, name, value):
        xpath.add_condition('%s = %s' % (name, self.xpath_literal(value)))
        return xpath

    def xpath_attrib_different(self, xpath, name, value):
        # FIXME: this seems like a weird hack...
        if value:
            xpath.add_condition('not(%s) or %s != %s'
                                % (name, name, self.xpath_literal(value)))
        else:
            xpath.add_condition('%s != %s'
                                % (name, self.xpath_literal(value)))
        return xpath

    def xpath_attrib_includes(self, xpath, name, value):
        if is_non_whitespace(value):
            xpath.add_condition(
                "%s and contains(concat(' ', normalize-space(%s), ' '), %s)"
                % (name, name, self.xpath_literal(' '+value+' ')))
        else:
            xpath.add_condition('0')
        return xpath

    def xpath_attrib_dashmatch(self, xpath, name, value):
        # Weird, but true...
        xpath.add_condition('%s and (%s = %s or starts-with(%s, %s))' % (
            name,
            name, self.xpath_literal(value),
            name, self.xpath_literal(value + '-')))
        return xpath

    def xpath_attrib_prefixmatch(self, xpath, name, value):
        if value:
            xpath.add_condition('%s and starts-with(%s, %s)' % (
                name, name, self.xpath_literal(value)))
        else:
            xpath.add_condition('0')
        return xpath

    def xpath_attrib_suffixmatch(self, xpath, name, value):
        if value:
            # Oddly there is a starts-with in XPath 1.0, but not ends-with
            xpath.add_condition(
                '%s and substring(%s, string-length(%s)-%s) = %s'
                % (name, name, name, len(value)-1, self.xpath_literal(value)))
        else:
            xpath.add_condition('0')
        return xpath

    def xpath_attrib_substringmatch(self, xpath, name, value):
        if value:
            # Attribute selectors are case sensitive
            xpath.add_condition('%s and contains(%s, %s)' % (
                name, name, self.xpath_literal(value)))
        else:
            xpath.add_condition('0')
        return xpath


class HTMLTranslator(GenericTranslator):
    """
    Translator for (X)HTML documents.

    Has a more useful implementation of some pseudo-classes based on
    HTML-specific element names and attribute names, as described in
    the `HTML5 specification`_. It assumes no-quirks mode.
    The API is the same as :class:`GenericTranslator`.

    .. _HTML5 specification: http://www.w3.org/TR/html5/links.html#selectors

    :param xhtml:
        If false (the default), element names and attribute names
        are case-insensitive.

    """

    lang_attribute = 'lang'

    def __init__(self, xhtml=False):
        self.xhtml = xhtml  # Might be useful for sub-classes?
        if not xhtml:
            # See their definition in GenericTranslator.
            self.lower_case_element_names = True
            self.lower_case_attribute_names = True

    def xpath_checked_pseudo(self, xpath):
        # FIXME: is this really all the elements?
        return xpath.add_condition(
            "(@selected and name(.) = 'option') or "
            "(@checked "
                "and (name(.) = 'input' or name(.) = 'command')"
                "and (@type = 'checkbox' or @type = 'radio'))")

    def xpath_lang_function(self, xpath, function):
        if function.argument_types() not in (['STRING'], ['IDENT']):
            raise ExpressionError(
                "Expected a single string or ident for :lang(), got %r"
                % function.arguments)
        value = function.arguments[0].value
        return xpath.add_condition(
            "ancestor-or-self::*[@lang][1][starts-with(concat("
                # XPath 1.0 has no lower-case function...
                "translate(@%s, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ', "
                               "'abcdefghijklmnopqrstuvwxyz'), "
                "'-'), %s)]"
            % (self.lang_attribute, self.xpath_literal(value.lower() + '-')))

    def xpath_link_pseudo(self, xpath):
        return xpath.add_condition("@href and "
            "(name(.) = 'a' or name(.) = 'link' or name(.) = 'area')")

    # Links are never visited, the implementation for :visited is the same
    # as in GenericTranslator

    def xpath_disabled_pseudo(self, xpath):
        # http://www.w3.org/TR/html5/section-index.html#attributes-1
        return xpath.add_condition('''
        (
            @disabled and
            (
                (name(.) = 'input' and @type != 'hidden') or
                name(.) = 'button' or
                name(.) = 'select' or
                name(.) = 'textarea' or
                name(.) = 'command' or
                name(.) = 'fieldset' or
                name(.) = 'optgroup' or
                name(.) = 'option'
            )
        ) or (
            (
                (name(.) = 'input' and @type != 'hidden') or
                name(.) = 'button' or
                name(.) = 'select' or
                name(.) = 'textarea'
            )
            and ancestor::fieldset[@disabled]
        )
        ''')
        # FIXME: in the second half, add "and is not a descendant of that
        # fieldset element's first legend element child, if any."

    def xpath_enabled_pseudo(self, xpath):
        # http://www.w3.org/TR/html5/section-index.html#attributes-1
        return xpath.add_condition('''
        (
            @href and (
                name(.) = 'a' or
                name(.) = 'link' or
                name(.) = 'area'
            )
        ) or (
            (
                name(.) = 'command' or
                name(.) = 'fieldset' or
                name(.) = 'optgroup'
            )
            and not(@disabled)
        ) or (
            (
                (name(.) = 'input' and @type != 'hidden') or
                name(.) = 'button' or
                name(.) = 'select' or
                name(.) = 'textarea' or
                name(.) = 'keygen'
            )
            and not (@disabled or ancestor::fieldset[@disabled])
        ) or (
            name(.) = 'option' and not(
                @disabled or ancestor::optgroup[@disabled]
            )
        )
        ''')
        # FIXME: ... or "li elements that are children of menu elements,
        # and that have a child element that defines a command, if the first
        # such element's Disabled State facet is false (not disabled)".
        # FIXME: after ancestor::fieldset[@disabled], add "and is not a
        # descendant of that fieldset element's first legend element child,
        # if any."