Void typed fields in union (aka set without a value)

It would be a nice addition to the language if thrift supported a void field type for unions, this would be used for fields that are set but without a value, i.e. the act of setting it is all the information that is required. Take this example:

union Query {
  // Find only this item
  1: string only;
  // Find any of these items
  2: list<string> anyOf;
  // Find all of these items
  3: list<string> allOf;
  // Every item
  4: void all;
}