@merge(map, map)
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, @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