instructions use a language called B<RPN> which is described in its
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.
+
=head1 DEF
B<DEF:>I<E<lt>vnameE<gt>>=I<E<lt>rrdfileE<gt>>:I<E<lt>ds-nameE<gt>>:I<E<lt>CFE<gt>>[:step=I<E<lt>stepE<gt>>][:start=I<E<lt>timeE<gt>>][:end=I<E<lt>timeE<gt>>][:reduce=I<E<lt>B<CF>E<gt>>]