theorem Th88:
  o,diff(b,a,o) // a,b
proof
  congr a,o,o,opposite(a,o) by Th81;
  then
A1: congr o,opposite(a,o),a,o;
  congr o,b,opposite(a,o),sum(b,opposite(a,o),o) by Def5;
  then congr o,opposite(a,o),b,diff(b,a,o) by Th69;
  then congr a,o,b,diff(b,a,o) by A1,Th65;
  then congr o,diff(b,a,o),a,b by Th69;
  hence thesis by Th57;
end;
