playground/coq/sigs.v

Definition X' : {a:nat & {b: nat | a < b}}.               
Proof.                                                   
  exists 2, 3.                                           
  constructor.
Defined.                                                 
                                                         
(* From Pierre Castéran *)
Definition X : {a:nat & {b: nat & {c: nat | a + b = c}}}.
Proof.                                                   
  now exists 3, 4, 7.                                    
Defined.