(** [shift path dt] shifts [path] by adding [dt] to all time values.
- val concat : 'a t list list -> 'a t list
-(** [concat ~t0 paths] concatenates [paths].
+ val concat : ?t0:float -> 'a t list list -> 'a t list
+(** Concatenation of time-stamped paths.
+ [concat ~t0 paths] concatenates [paths] as if they were ordinary lists but
+ time shifts each path to align with the previous path.
+ The first path is time shifted by [~t0] ([0.0] by default). The next path is
+ time shifted by the time of the last configuration of the previously shifted