moduuuuuuuleeeeeeeeeeeeee

module A
  export type Base
  export val global.f : Base -> Base -> Ret # vtbl 作りたいけど、作れない

module B
  export type X can cast from Base
  export val global.f : X -> Base -> Ret

module C
  export type Y can cast from Base
  export val global.f : Base -> Y -> Ret

module D
  import B, C
  call global.f x, y # conflict!!!

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module A
  export abstract type Base
  export virtual method global.f : Base -> Ret # vtbl 作りたいけど、作れない

module B
  export type X <: Base
  export virtual method global.f : X -> Ret

module C
  export type Y <: Base
  export virtual method global.f : Y -> Ret