* different boundary indicators and call this function twice, once for each
* boundary indicators; doing so will yield only one normal vector at this
* point per invocation (because we consider only one boundary part at a
- * time), with the result that the normal vectors will not be averaged.
+ * time), with the result that the normal vectors will not be averaged. This
+ * situation also needs to be taken into account when using this function
+ * around reentrant corners on Cartesian meshes. If no-normal-flux boundary
+ * conditions are to be enforced on non-Cartesian meshes around reentrant
+ * corners, one may even get cycles in the constraints as one will in
+ * general constrain different components from the two sides. In that case,
+ * set a no-slip constraint on the reentrant vertex first.
*
*
* <h4>Computing constraints in 3d</h4>