reserve fi,psi for Ordinal-Sequence,
  A,A1,B,C,D for Ordinal,
  X,Y for set,
  x,y for object;

theorem
  A+^B in C implies B in C-^A
proof
A1: A+^B-^A = B by Th52;
  A c= A+^B by Th24;
  hence thesis by A1,Th53;
end;
