Existential 1 =
?Goal1 : [p : nat  q := S p : nat  n : nat  m : nat |- ?y = m] 
Existential 2 =
?y : [p : nat  q := S p : nat  n : nat  m : nat |- nat] (p, q cannot be used)
Existential 3 = ?Goal : [q : nat  n : nat  m : nat |- n = ?y]