@gshift_map(a:map, f:function)
(where f can be a map, a nim or an intentional function), builds a new map b such that 
       b(f(x)) = a(x)

See also Map (aka dictionnary) @add_pair,    @clear,    @count,    @domain,    @find,    @gshift_map,    @insert,    @is_defined,    @is_function,    @is_map,    @listify,    @make_duration_map,    @make_label_bpm,    @make_label_duration,    @make_label_pitches,    @make_label_pos,    @make_score_map,    @map,    @map_compose,    @map_concat,    @map_history,    @map_history_date,    @map_history_rdate,    @map_normalize,    @map_reverse,    @mapval,    @max_key,    @max_val,    @member,    @merge,    @min_key,    @min_val,    @occurs,    @range,    @remove,    @select_map,    @shift_map