theorem Th73:
  congr b,a,p,q & congr c,a,p,r implies b,c // q,r
proof
  assume congr b,a,p,q & congr c,a,p,r;
  then congr b,c,r,q by Th71;
  then b,c // r,q by Th57;
  hence thesis by Th6;
end;
