Add another note about the BR element.

diff --git a/9.1/paper.tex b/9.1/paper.tex
index a53841e94528bbe3edad4c89715954af88d5a6d6..49a37b4b48583edbbd7a257cd597c38ed42016cd 100644 (file)
--- a/9.1/paper.tex
+++ b/9.1/paper.tex
@@ -173,6 +173,17 @@ following section. There are a number of other noteworthy changes in this releas
   pressure space to provide additional accuracy commensurate with the
   cost of the large number of velocity unknowns. The BR space is
   intermediate to the $Q_1^d$ and $Q_2^d$ spaces.
+
+  The current implementation of the element only works for
+  parallelogram meshes due to difficulties associated with the mapping
+  of shape functions: The shape functions of the $Q_1^d$ part of the
+  element need to be mapped as scalars, as is common for the vector
+  components of the $Q_1^d$ element; on the other hand, the
+  vector-valued edge bubble functions need to be mapped using the
+  Piola transform as is common for the Raviart-Thomas
+  element. \dealii{} does not currently have the ability to use
+  different mappings for individual shape functions, though this
+  functionality is planned for the next release.
   
 \item The \texttt{FE\_NedelecSZ} class is an alternative
   implementation of the ideas behind the N{\'e}d{\'e}elec element. It is based