assert value == null : Debug.show(symbol);
assert value == null : Debug.show(symbol) + " -> " + value;
    public Override insertOverride(Symbol symbol, Override override) {        assert symbol.isTerm() : Debug.show(symbol);        assert Debug.log("insert override: ", symbol);        Object value = overrides.put(symbol, override);        assert value == null : Debug.show(symbol);        return override;    }