code: func AttributeExists(nameBuilder NameBuilder) ConditionBuilder {
  	return ConditionBuilder{
  		operandList: []OperandBuilder{nameBuilder},
  		mode:        attrExistsCond,
  	}
  }
docs: // AttributeExists returns a ConditionBuilder representing the result of the
  // attribute_exists function in DynamoDB Condition Expressions. The resulting
  // ConditionBuilder can be used as a part of other Condition Expressions or as
  // an argument to the WithCondition() method for the Builder struct.
  //
  // Example:
  //
  //     // condition represents the boolean condition of whether the item
  //     // attribute "Age" exists or not
  //     condition := expression.AttributeExists(expression.Name("Age"))
  //
  //     // Used in another Condition Expression
  //     anotherCondition := expression.Not(condition)
  //     // Used to make an Builder
  //     builder := expression.NewBuilder().WithCondition(condition)
  //
  // Expression Equivalent:
  //
  //     expression.AttributeExists(expression.Name("Age"))
  //     "attribute_exists (Age))"