--- a/src/lib/quaternion.h
+++ b/src/lib/quaternion.h
@@ -70,6 +70,8 @@
 	return sqr(q.R_component_1()) + sqr(q.R_component_2()) + sqr(q.R_component_3()) + sqr(q.R_component_4());
 }
 
+bool quaternion_is_normalized(const qt& q);
+
 inline void quaternion_normalize(qt& q) {
 	const fl s = quaternion_norm_sqr(q);
 	assert(eq(s, sqr(boost::math::abs(q))));