src/HOL/Integ/Lagrange.ML

 \  (sq x1 + sq x2 + sq x3 + sq x4) * (sq y1 + sq y2 + sq y3 + sq y4) = \
 \  sq(x1*y1 - x2*y2 - x3*y3 - x4*y4)  + \
 \  sq(x1*y2 + x2*y1 + x3*y4 - x4*y3)  + \
 \  sq(x1*y3 - x2*y4 + x3*y1 + x4*y2)  + \
 \  sq(x1*y4 + x2*y3 - x3*y2 + x4*y1)";
-by(cring_simp 1);
+(*Takes up to three minutes...*)
+by (cring_simp 1);
 qed "Lagrange_lemma";