theorem
  for a,b,c,X being set st a in X & b in X & c in X holds {a,b,c} c= X
proof
  let a,b,c,X be set;
  assume a in X & b in X & c in X;
  then {a,b} c= X & {c} c= X by Lm1,Th31;
  then {a,b} \/ {c} c= X by XBOOLE_1:8;
  hence thesis by ENUMSET1:3;
end;
