'Traverse an AST, applying a set of fixers to each node.
This is a helper method for refactor_tree().
Args:
fixers: a list of fixer instances.
traversal: a generator that yields AST nodes.
Returns:
None'

def traverse_by(self, fixers, traversal):

    if (not fixers):
        return
    else:
        for node in traversal:
            for fixer in fixers[node.type]:
                results = fixer.match(node)
                if results:
                    new = fixer.transform(node, results)
                    if (new is not None):
                        node.replace(new)
                        node = new
        return
