lookup(name) {
    var val;
    if (this.ignoreColors && name in colors) return;
    if (val = this.stack.lookup(name)) {
      return utils.unwrap(val);
    } else {
      return this.lookupFunction(name);
    }
  }
Lookup `name`, with support for JavaScript
functions, and BIFs.

@param {String} name
@return {Node}
@api private