Flanking fire is clearly based on the cross-fire rule from PG, Unlike AT fire, lines of infantry work differently. This becomes a problem, as came up at a bridge on Sunday:
The first group fires on the Spaniards in one activation. The next American activation has the captain activates his stack and the one next to him in a combined fire group. Should they get the flank fire bonus? The the southern most Americans are clearly firing through a non-adjacent hexside. Does the northern component of the fire group really prevent enfilade fire?
We resolved the question of the moment with a coin flip. The result being the modifier was not applied for that session.