'Turns the /var/value prefix into a query-string argument.
Returns a new path with the prefix stripped.
>>> query_data = {}
>>> path = urlmap._prefix_to_query(\'/var/val/stuff\', query_data, {})
>>> path, query_data
(\'/stuff\', {\'var\': [\'val\']})'

def _prefix_to_query(self, path, query_data, post_data):

    (which, value, path) = path[1:].split('/', 2)
    query_data[which] = [value]
    return ('/' + path)
