Added clearenv().

Author: janpgit

intro.tex

@@ -2022,7 +2022,8 @@
     environment (adds a new or modifies an existing variable)
     \end{itemize}
 \item changes are propagated into children upon their creation
-\item there are also functions \funnm{setenv}() and \funnm{unsetenv}()
+\item there are also functions \funnm{setenv}(), \funnm{unsetenv}(), and
+\funnm{clearenv}()
 \end{itemize}
 \end{slide}