constructor(e, t) {
    e === void 0 && (e = {}), t === void 0 && (t = {}), this.current = void 0, this.builtins = void 0, this.undefStack = void 0, this.current = t, this.builtins = e, this.undefStack = [];
  }
Detect whether `name` has a definition.  Equivalent to
`get(name) != null`.