I _think_ (but unsure) that it should be read “followed by”, and only makes sense in the context of a while loop, where `x = y fby z` means that x will take the value of y on the first iteration and z on subsequent iterations.
You're right, the name was borrowed from Lucid, where it is a proper operator on streams.
side note: Tic-Tac-Toe you can reasonably just brute-force all possible games to check if something plays perfect or not.
I know, the point was to exercise the various PyFL features.
Actually, the first moves of the players determines the result. And because of the symetries there are just 12 first moves.