reserve f for Function;
reserve p,q for FinSequence;
reserve A,B,C for set,x,x1,x2,y,z for object;
reserve k,l,m,n for Nat;
reserve a for Nat;

theorem Th6:
  f just_once_values y iff ex x st {x} = f " {y}
by CARD_2:42;
