Conforming types may be used in for-in statements.

SequenceType.swift

extension <# extendedType #> : SequenceType {  
  func generate() -> <# generatorType #><<# elementType #>> {
    <# generator state #>

    return anyGenerator {
      if <# terminal condition #> {
        return nil
      }

      <# iteration logic #>

      return nil
    }
  }
}