@merge(map, map)
returns a new map which is the asymetric merge of the two argument maps.

The result of @merge(a, b) is a map c such that c(x) = a(x) if a(x) is defined, and b(x) elsewhere.

Notice that a(x) is defined if x is a key in a but the value a(x) may be the undef value.

See also @map_compose and @mapval.

See also Map (aka dictionnary) @add_pair    @clear    [@clone]    @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