Because the RHS can return a delivery-agent specification, it is
sometimes desirable to check for the $# operator
on the LHS of a rule. Consider, for example, the following rule:
R $+ $| $# OK $@ $1
The LHS looks for anything (the $+) followed by a
$| operator, and then a $# OK.
This might match a workspace that was set up by a database-map lookup
or a call to another rule set. The $# OK means the
address was OK as is, and so should be placed back into the
workspace. The RHS does just that by returning (the
$@ prefix) the original address (the
$1 references the LHS $+, which
contained the original address).
Note that the $# operator has no special meaning
in the LHS. It is used only to detect a delivery-agent-like
specification made by an earlier rule on the RHS. The next two
sections reveal how this is done.