It is often valuable to group rule sets by function and call them as
subroutines from a rule. To illustrate, consider the following rule:
R $+ < @ $+ > $: $>set
Here, the RHS $>set
operator tells sendmail to perform additional
rewriting using a secondary set of rules called
set. The workspace is passed as is to that
secondary rule set, and the result of the rewriting by that secondary
rule set becomes the new workspace.
The $> operator is described in full in Section 18.7.4.