Get the value of the simple content of this element.
        Parameters:
            elt -- the DOM element being parsed
            ps -- the ParsedSoap object.
            mixed -- ignore element content, optional text node

def simple_value(self, elt, ps, mixed=False):
        '''Get the value of the simple content of this element.
        Parameters:
            elt -- the DOM element being parsed
            ps -- the ParsedSoap object.
            mixed -- ignore element content, optional text node
        '''
        if not _valid_encoding(elt):
            raise EvaluateException('Invalid encoding', ps.Backtrace(elt))
        c = _children(elt)
        if mixed is False:
            if len(c) == 0:
                raise EvaluateException('Value missing', ps.Backtrace(elt))
            for c_elt in c:
                if c_elt.nodeType == _Node.ELEMENT_NODE:
                    raise EvaluateException('Sub-elements in value',
                        ps.Backtrace(c_elt))

        # It *seems* to be consensus that ignoring comments and
        # concatenating the text nodes is the right thing to do.
        return ''.join([E.nodeValue for E in c
                if E.nodeType 
                in [ _Node.TEXT_NODE, _Node.CDATA_SECTION_NODE ]])