mapping filter_indices(mapping, string func, string|object ob, ...)
mapping filter_indices(mapping, closure cl, ...)
ob->func() is called resp. cl applied to every element in the
mapping, with first argument being the key of the
element, and then the extra args that were given to
filter_indices (these args must not be protected references like
&(i). If the function returns true, the element is
added to the result mapping. ob can also be an object_name of an
If <ob> is omitted, or neither a string nor an object, it
defaults to this_object().
In LDMud 3.2.6 renamed from filter_mapping() and generalised
by efun filter().
filter(E), map(E), map_indices(E), walk_mapping(E), member(E),