keep_if()
Deletes every key-value pair from hsh for which block evaluates to false.
If no block is given, an enumerator is returned instead.
VALUE rb_hash_keep_if(VALUE hash) { RETURN_SIZED_ENUMERATOR(hash, 0, 0, hash_enum_size); rb_hash_modify_check(hash); if (RHASH(hash)->ntbl) rb_hash_foreach(hash, keep_if_i, hash); return hash; }