theorem Th85:
  p,q // sum(p,r,o),sum(q,r,o)
proof
  congr o,p,r,sum(p,r,o) by Def5;
  then
A1: congr p,o,sum(p,r,o),r by Th69;
  congr o,q,r,sum(q,r,o) by Def5;
  then congr p,q,sum(p,r,o),sum(q,r,o) by A1,Th70;
  hence thesis by Th57;
end;
