theorem Th179:
  A c= B iff A--a c= B--a
proof
  thus A c= B implies A--a c= B--a by Th67;
  assume
A1: A--a c= B--a;
  let z;
  assume z in A;
  then z-a in A--a by Th176;
  then ex c st z-a = c-a & c in B by A1,Th178;
  hence thesis;
end;
