astIdentifierExpression(idtNode, retArr) {
    if (idtNode.type !== 'Identifier') {
      throw this.astErrorOutput(
        'IdentifierExpression - not an Identifier',
        idtNode
      );
    }

    switch (idtNode.name) {
      case 'Infinity':
        retArr.push('Infinity');
        break;
      default:
        if (this.constants && this.constants.hasOwnProperty(idtNode.name)) {
          retArr.push('constants_' + idtNode.name);
        } else {
          retArr.push('user_' + idtNode.name);
        }
    }

    return retArr;
  }
@desc Parses the abstract syntax tree for *identifier* expression
@param {Object} idtNode - An ast Node
@param {Array} retArr - return array string
@returns {Array} the append retArr