class C;
  rand int x;
  constraint proto1;
  extern constraint proto2;
endclass

class A;
  rand C c;
  rand int y;
  constraint proto3 {
    y == 0;
  }
  function new();
    c = new;
  endfunction
endclass

// if the constraint body "C::proto1" is not defined, VCS issues a warning
// and treats it as an empty constraint
constraint C::proto1 { x < 10; }

// if the constraint body "C::proto2" is not defined, VCS issues an error
constraint C::proto2 { x > 4; }

如果聲明了 external，但是外面沒有定義該constraint，會報error。
如果不是external的，沒有定義，報warning，認為是個空約束。