_toString (options) {
      const parenthesis =
          (options && options.parenthesis) ? options.parenthesis : 'keep'
      let expr = this.expr.toString(options)
      if (needParenthesis(this, parenthesis, options && options.implicit)) {
        expr = '(' + expr + ')'
      }
      return this.name + '(' + this.params.join(', ') + ') = ' + expr
    }
get string representation
@param {Object} options
@return {string} str