@value interface Base<|#x> {}

@value interface Value<#x|#y|#z> {
  refines Base<#self>
  call () -> (#self)
}

concrete Child1 {
  refines Value<all,all,all>
  @value call () -> (#self)
}

concrete Child2 {
  refines Value<all,all,all>
  @value call () -> (all)
}