Deferred choice: the first branch whose wait goes on wins

[noha]

FBDeferredChoice is a split whose branches each lead into a waiting spot - a condition wait, a trigger target, a remote flow, a sub flow call - and of which exactly one goes on: the first one whose wait is resumed. The others fall the moment the choice is decided, queued or parked, standing in a loop or a sub flow, in this instance or one executing inside it. An FBDeferredChoiceDecided records the winner.

The decision is made where a wait goes on, in the one continuation path FBBaseExecutor>>resume:withToken:value:, before the wait is taken out; a condition wait whose condition holds at once and a sub flow call that runs through decide it as well. A pass remembers its origin token and whether it was decided. A branch that could not wait is refused when the choice runs.

A withdrawal now takes the tokens of standing activations too - a fork whose loop is open, a sub flow node whose child runs - when they lie on the strand taken back: the loop starts nothing more and the frame pops quietly, the call reading #cancelled. cancelStrand: into a sub flow thus ends the call instead of failing the caller.