theorem Th66:
  a in Tarski-Class(a\/b\/c)
  proof set U = Tarski-Class(a\/b\/c);
    a c= a\/b & a\/b c= a\/b\/c by XBOOLE_1:7; then
    a c= a\/b\/c & a\/b\/c in U by CLASSES1:2;
    hence thesis by CLASSES1:def 1;
  end;
