Run two processes in parallel exposing the free ports of both of them.

The typeclass constaint Forkable here ensures that:

1. No more than one of forked processes expects to finish in NextSend.
2. Only the process that finishes in NextSend is allowed to produce a (non-Void) result.

The resulting action that implements the fork_ starts in NextSend if one of the forked actions started in NextSend. Similarly, it finishes in NextSend if one of the forked two finishes in NextSend. (Implemented with ForkIndexOr.)

The return value of the fork_ is the same as return value of process that finishes in NextSend. If both finish in NextRecv, then fork_'s return value is Void.

Swap two free ports.

We go from an action where free ports are p ++ [f] ++ p' ++ [s] ++ rest to p ++ [s] ++ p' ++ [f] ++ rest. In other words, the elemens f and s are swapped.

The ListSplitD arguments can often be derived automatically if p p' and rest are fully known.

Link two adjacent free ports with each other. This binds them and removes from the free list.

Reorders the open the free ports.

The permPorts f g should only be called with f . g = id. This is not verified by compiler, should be checked manually.

For a safe alternative, see swap_. That one is guaranteed to only permute ports and never cause bad behaviors.