reserve A for RelStr;
reserve X for non empty set;
reserve PX,PY,PZ,Y,a,b,c,x,y for set;
reserve S1,S2 for Subset of Y;

theorem Th9:
  {a} is mutually-disjoint
proof
  let x,y be set such that
A1: x in {a} and
A2: y in {a} and
A3: x <> y;
  x = a by A1,TARSKI:def 1;
  hence thesis by A2,A3,TARSKI:def 1;
end;
