##
## Extract the value of the named KEY in the ${extract ...}-style string in
## the current $value (where it may have been left by `LOOKUP-DOMAIN'). The
## RESULT is a pair {IF-FOUND}{IF-NOT-FOUND}; the former may use $value to
## refer to the value; the latter may be `fail' (not in braces) to force
##
## Extract the value of the named KEY in the ${extract ...}-style string in
## the current $value (where it may have been left by `LOOKUP-DOMAIN'). The
## RESULT is a pair {IF-FOUND}{IF-NOT-FOUND}; the former may use $value to
## refer to the value; the latter may be `fail' (not in braces) to force