@classmethod
def _UseForTag(cls, tag, raise_if_fail=True):
    """Return the ElementDeclaration object corresponding to the element name.

        @param tag: The L{ExpandedName} of an element in the class."""
    try:
        rv = cls._ElementMap[tag]
    except KeyError:
        if raise_if_fail:
            raise
        rv = None
    return rv
