Set of symbols required by this Require

        :return: set of ``(symbol type, symbol name)`` tuples

def as_symbols(self):
        """Set of symbols required by this Require

        :return: set of ``(symbol type, symbol name)`` tuples
        """
        out = set()
        for name in self.types:
            out.add(('type', name))
        for name in self.enums:
            out.add(('enum', name))
        for name in self.commands:
            out.add(('command', name))
        return out