Derivers for control tracks. That means tempo, control, and pitch.

Control and pitch tracks can have a Merger. If no merger is given, they are combined with get_default_merger. set will replace the signal. So two tracks named c will multiply, same as if the second were mul c. If you want to override c then set c will do that.

Tempo tracks don't support mergers because they are converted into a warp, which is then combined via composition. Pitch tracks normally replace each other because adding together absolute pitches is undefined. Relative pitches can be added or multiplied, and this is expressed via normal controls using transposition signals like Controls.chromatic.