reserve n for non zero Element of NAT;
reserve a,b,r,t for Real;

theorem Th2:
  for x,y,z being set st x is_a_fixpoint_of {[y,z]} holds x = y
  proof
    let x,y,z be set;
    assume
A1: x in dom {[y,z]};
    dom {[y,z]} = {y} by RELAT_1:9;
    hence thesis by A1,TARSKI:def 1;
  end;
