own manual page.
Variable names (I<vname>) must be made up strings of the following characters
-C<A-Z, a-z, 0-9, -,_>, starting with a letter and a maximum length of 255 characters.
+C<A-Z, a-z, 0-9, -,_> and a maximum length of 255 characters.
=head1 DEF
Example: C<VDEF:avg=mydata,AVERAGE>
+Note that currently only agregation functions work in VDEF rpn expressions.
+Patches to change this are welcome.
+
=head1 CDEF
B<CDEF>:I<vname>=I<RPN expression>