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

theorem
  C <> {} & A in B+^C implies A-^B in C
proof
  assume
A1: C <> {};
A2: B+^C-^B = C by Th52;
  not B c= A implies A-^B = {} by Def5;
  hence thesis by A1,A2,Th8,Th53;
end;
