§4 · channel composition

Two epigenetic channels write one drive: add, veto, and path-dependence

Methylation and small RNA write the same drive on one switch, so they compose by sign: same-sign writes ADD and cross the spinodal together, opposite-sign writes VETO and cancel, and because the switch is hysteretic the order of writes latches the outcome — a memory of which channel won. [V].

On REC8 (γ = 1.4525, spinodal 0.6738) neither channel flips alone but the same-sign sum does; an opposing write vetoes a flip that either channel would otherwise make; and on PAX5 (γ = 1.4892) a sub-spinodal opposite write cannot undo a state that already crossed.

Same sign: the channels add

On REC8 (spinodal 0.6738), methylation alone is below threshold and RNA alone is below threshold, but written together with the same sign their sum crosses the spinodal and the switch flips. Two sub-threshold writes make one supra-threshold drive.

Opposite sign: the channels veto

At the same locus, a methylation write that would flip the switch ON is held OFF by an opposing RNA write of comparable size. The drives cancel on the shared field; neither edits the ruler.

Order latches the outcome

Because the switch is hysteretic, the result depends on the order, not just the sum. On PAX5 (spinodal 0.6995), a channel that crosses first and then meets a sub-spinodal opposite write stays ON; a sequence that never crosses stays OFF. The combined state remembers which channel won — the substrate of an order-dependent epigenetic history.