Transient milestones are milestones with a degree of two (if undirected) or with an indegree of one and outdegree of one (if directed)
simplify_trajectory(trajectory, allow_self_loops = FALSE)
Whether or not to allow self loops.
The positions of the cells within the trajectory remain the same
A -> B -> C is simplified to A -> C.
A — B — C is simplified to A — C.
A -> B -> C -> D -> A is simplified to A -> B -> D.