add OnceMap that runs an IO action once for each distinct argument it is passed.
add OnceMap that runs an IO action once for each distinct argument it is passed.

darcs-hash:20051013094852-1a7c6-5846e2c70751b4dc5f574d992ec2489fc6f5721c

kirbyfan64/jhc,kirbyfan64/jhc,kirbyfan64/jhc,kirbyfan64/jhc