reserve a,b,c,d for Ordinal;
reserve l for non empty limit_ordinal Ordinal;
reserve u for Element of l;
reserve A for non empty Ordinal;
reserve e for Element of A;
reserve X,Y,x,y,z for set;
reserve n,m for Nat;
reserve f for Ordinal-Sequence;
reserve U,W for Universe;
reserve F,phi for normal Ordinal-Sequence of W;
reserve g for Ordinal-Sequence-valued Sequence;

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;
