assert value == null : Debug.show(symbol);
assert value == null : Debug.show(symbol) + " -> " + value;
    public Function insertFunction(Symbol symbol, Function function) {        assert symbol.isTerm() : Debug.show(symbol);        assert Debug.log("insert function: ", symbol);        Object value = functions.put(symbol, function);        assert value == null : Debug.show(symbol);        return function;    }