@type interface Type<#a,#b|#c,#d|#e,#f> {
  #a allows Parent
  #b requires Type2<#a>
  #c defines Equals<#c>

  create () -> (optional #x)

  create2<#y>
    #y requires #x
    #y allows T2
    #y defines T3
  () -> (optional #x)
}