Actually, it is just $1 (in mIRC terms). $calc is an identifier, therefore $1 $2 etc are populated by the comma-delimited arguments within the () of $calc().
Example:
alias idtest { echo -a 1: $1 | echo -a 2: $2 }
//noop $idtest(1 + 2)
$calc only supports one argument (the equation). If you add a comma (to populate $2 $3 etc) you will get an invalid format error.
-genius_at_work
