reserve A,A1,A2,B,B1,B2,C,O for Ordinal,
      R,S for Relation,
      a,b,c,o,l,r for object;

theorem Th30:
  A c= B implies ClosedProd(R,A,A) c= ClosedProd(R,B,B)
proof
  assume A c= B;
  then A=B or A in B by ORDINAL1:11,XBOOLE_0:def 8;
  hence thesis by Th17;
end;
