<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def from_string(cls, s):
 """ Instantiate a `Derivation` from a UDF or UDX string representation. The UDF/UDX representations are as output by a processor like the `LKB <http://moin.delph-in.net/LkbTop>`_ or `ACE <http://sweaglesw.org/linguistics/ace/>`_, or from the :meth:`UdfNode.to_udf` or :meth:`UdfNode.to_udx` methods. Args: s (str):
 UDF or UDX serialization """

        if not (s.startswith('(') and s.endswith(')')):
            raise ValueError(
                'Derivations must begin and end with parentheses: ( )'
            )
        s_ = s[1:]  # get rid of initial open-parenthesis
        stack = []
        deriv = None
        try:
            matches = cls.udf_re.finditer(s_)
            for match in matches:
                if match.group('done'):
                    node = stack.pop()
                    if len(stack) == 0:
                        deriv = node
                        break
                    else:
                        stack[-1].daughters.append(node)
                elif match.group('form'):
                    if len(stack) == 0:
                        raise ValueError('Possible leaf node with no parent.')
                    gd = match.groupdict()
                    # ignore LKB-style start/end data if it exists on gd
                    term = UdfTerminal(
                        _unquote(gd['form']),
                        tokens=_udf_tokens(gd.get('tokens')),
                        parent=stack[-1] if stack else None
                    )
                    stack[-1].daughters.append(term)
                elif match.group('id'):
                    gd = match.groupdict()
                    head = None
                    entity, _, type = gd['entity'].partition('@')
                    if entity[0] == '^':
                        entity = entity[1:]
                        head = True
                    if type == '':
                        type = None
                    udf = UdfNode(gd['id'], entity, gd['score'],
                                  gd['start'], gd['end'],
                                  head=head, type=type,
                                  parent=stack[-1] if stack else None)
                    stack.append(udf)
                elif match.group('root'):
                    udf = UdfNode(None, match.group('root'))
                    stack.append(udf)
        except (ValueError, AttributeError):
            raise ValueError('Invalid derivation: %s' % s)
        if stack or deriv is None:
            raise ValueError('Invalid derivation; possibly unbalanced '
                             'parentheses: %s' % s)
        return cls(*deriv, head=deriv._head, type=deriv.type)