code: function compileToken (token, arg) {
    let fn = tokens[token] || noOp
  
    return function () {
      return arg ? fn.call(this, arg) : fn.call(this)
    }
  }
docs: Compile a token string into a function.
  
  @param {string} token
  @param {string} arg
  @return {function}
  @private