theorem Th2:
  i divides a & i divides (a-b) implies i divides b
proof
  assume that
A1: i divides a and
A2: i divides (a-b);
A3: b=-(a-b)+a;
  i divides -(a-b) by A2,INT_2:10;
  hence thesis by A1,A3,WSIERP_1:4;
end;
