macro "foo" x:ident : command =>
  `(structure Foo where
      val : Nat
      prop : val = 42
    def f (x : Foo) := x.val
    def g : Foo := { val := 42, prop := by decide }
    theorem $x:ident (x : Foo) : f x = 42 := by simp [f, x.prop] )

foo test
#print test