message: Add comment explaining lexicographic ordering
diff: @@ -734,10 +734,24 @@ public:
  DenseMatrix &div) const;
  /** @brief Get an Array<int> that maps lexicographically ordered indices to
  - the indices of the respective nodes/dofs/basis functions. The returned
  - array may be empty if the DOFs are already ordered lexicographically, or
  - if the finite element does not support creating this permutation. The
  - array returned is the same as the array given by
  + the indices of the respective nodes/dofs/basis functions. Lexicographic
  + ordering of nodes is defined in terms of reference-space coordinates
  + (x,y,z). Lexicographically ordered nodes are listed first in order of
  + increasing x-coordinate, and then in order of increasing y-coordinate,
  + and finally in order of increasing z-coordinate.
  +
  + For example, the six nodes of a quadratic triangle are lexicographically
  + ordered as follows:
  +
  + 5
  + |\
  + 3 4
  + | \
  + 0-1-2
  +
  + The resulting array may be empty if the DOFs are already ordered
  + lexicographically, or if the finite element does not support creating
  + this permutation. The array returned is the same as the array given by
  TensorBasisElement::GetDofMap, but it is also available for non-tensor
  elements. */
  const Array<int> &GetLexicographicOrdering() const { return lex_ordering; }
  