* Sort the degrees of freedom by
* block. It does the same
* thing as the above function.
+ *
+ * This function only succeeds if each of
+ * the elements in the hp::FECollection
+ * attached to the hp::DoFHandler argument
+ * has exactly the same number of blocks
+ * (see @ref GlossBlock "the glossary" for
+ * more information). Note that this is not
+ * always given: while the hp::FECollection
+ * class ensures that all of its elements
+ * have the same number of vector
+ * components, they need not have the same
+ * number of blocks. At the same time, this
+ * function here needs to match individual
+ * blocks across elements and therefore
+ * requires that elements have the same
+ * number of blocks and that subsequent
+ * blocks in one element have the same
+ * meaning as in another element.
*/
template <int dim>
void