theorem Th68:
  for X1,X2,Y be set holds
    comp(X1\/X2,x,y,Y) = comp(X1,x,y,Y)\/comp(X2,x,y,Y)
proof
  let X1,X2,Y be set;
  comp(X1\/X2,x,y,Y) = comp(Y,y,x,X1\/X2) by Th53
  .= comp(Y,y,x,X1) \/comp(Y,y,x,X2) by Th60
  .= comp(X1,x,y,Y) \/comp(Y,y,x,X2) by Th53;
  hence thesis by Th53;
end;
