Add details of fp var/fn addition

base/doc/ltnews38.tex

@@ -260,6 +260,38 @@ \subsection{In the programming layer}
 the input, or to titlecase every word. This should be transparent at the
 document level but will be useful for programmers.
 
+We have also added the ability to define variables and functions inside
+\cs{fpeval} (at the \pkg{expl3} level this is \cs{fp\_eval:n}). This allows
+programmers to create non-standard functions that can then be used inside
+\cs{fpeval}. For example, this could be used to create a a new function
+\texttt{dinner}:
+\begin{verbatim}
+\documentclass{article}
+\begin{document}
+\ExplSyntaxOn
+\fp_new_variable:n{duck}
+\fp_new_function:n{dinner}
+\fp_set_function:nnn{dinner}{duck}{duck-0.25duck}
+\fp_set_variable:nn{duck}{1}
+$\fp_eval:n{duck}
+ >\fp_eval:n{dinner(duck)}
+  \fp_set_variable:nn{duck}{dinner(duck)}
+ >\fp_eval:n{dinner(duck)}
+  \fp_set_variable:nn{duck}{dinner(duck)}
+ >\fp_eval:n{dinner(duck)}
+  \fp_set_variable:nn{duck}{dinner(duck)}
+ >\fp_eval:n{dinner(duck)}
+$
+\ExplSyntaxOff
+\end{document}
+\end{verbatim}
+
+Users will be able to access added functions without needing to use the
+\pkg{expl3} layer. It is possible that a future release of \LaTeX{} will
+add the ability to create and set floating point variables to the
+document level: this will be examined based on feedback on the
+utility of the programming layer change.
+
 \section{Bug fixes}
 
 \section{Changes to packages in the \pkg{amsmath} category}