Synopsys   : FILTER_PRESENT(x)

Defined in : /sys/filter_funs.h

Description: This filter returns all elements of an array that are in the
             environment of this_player().
Arguments  : object *x - the array with objects to filter.
Returns    : object *  - an array with only objects that are in the
                         environment of this_player().

Caveat     : All items in the array x _must_ be objects. The macro is _not_
             forgiving to elements 0.
