When looking up information or performing actions with the $( and $) operators, it is sometimes necessary to provide positional substitution arguments. To illustrate, consider an entry such as this in a hypothetical database source file:

hostA   %0!%1@%2

With such an entry in place, and having built the database, the following rule could be used to perform a lookup:

R$- @ $-.uucp   $: $(uucp $2 $@ $1 $@ mailhost $: $1.$2.uucp $)

Here, if the workspace contains the address joe@hostA.uucp, the LHS matches, causing it to be rewritten as hostA!joe@mailhost.

See Section 23.4.2 for a full description of how $@ is used in this way.