'Returns the transformation for a given parse tree node.
Args:
node: the root of the parse tree that matched the fixer.
results: a dict mapping symbolic names to part of the match.
Returns:
None, or a node that is a modified copy of the
argument node.  The node argument may also be modified in-place to
effect the same change.
Subclass *must* override.'

def transform(self, node, results):

    raise NotImplementedError()
