theorem Th70:
  congr a,b,p,q & congr b,c,q,s implies congr a,c,p,s
proof
  assume congr a,b,p,q & congr b,c,q,s;
  then congr b,q,a,p & congr b,q,c,s by Th69;
  then congr a,p,c,s by Th65;
  hence thesis by Th68;
end;
