'Extract the keys from all the <var>s in the node.'

def process_vars(self, fixable_vars):

    match_keys = set()
    for var_node in fixable_vars:
        match = self.get_match(var_node)
        if match:
            key = self.extract_key(match)
            if key:
                match_keys.add(key)
    self.match_keys = list(match_keys)
    return super(IfElseFilter, self).process_vars(fixable_vars)
