Toilet flushing instructions and recursive binary Merge

Spotted in the Duke of Cumberland loos in Whitstable this weekend, this instruction on how to flush the toilet:

Press hard
Both buttons

I interpret this as two instructions:

PRESS HARD. BOTH BUTTONS. 

You could read it as a single instruction ('Press hard both buttons') but it's awkward in English. It would be normal in, say, Spanish, but in English a more idiomatic word order is 'Press both buttons hard', or verb (press) – object (both buttons) – adverb (hard).

The way that this is broken down into two phrases can be seen as support for the idea that syntax comprises a series of operations of recursive binary Merge. That's a technical way of saying that sentences are formed by combining two elements at a time, and combining the resulting component with a new element, still two at a time.

So, for example, we might think of a sentence like Birds eat seeds as being formed as follows:

eat + seeds --> eat seeds
birds + eat seeds --> birds eat seeds

Our loo-flushing example is a little bit more complicated. We don't have a subject, because it's an instruction so there is an implicit 'you' as the presser of the buttons. We definitely want to say that both buttons is a unit (a 'constituent'), which seems intuitively right (there are also ways to test this kind of thing). Then we want to say that Press both buttons is a constituent, with the verb press combining with its object both buttons. Then we would combine that whole phrase press both buttons with the adverb hard, telling us how the action of pressing both buttons should be performed. This makes more sense than saying that the verb press combines with a constituent both buttons hard, which doesn't seem intuitively right. Adverbs tell us how verbs are done, not what nouns are like. So now we have this structure:

both + buttons --> both buttons
press + both buttons --> press both buttons
press both buttons + hard --> press both buttons hard

The fact that the adverb refers to the verb, and not to the noun, also tells us why we get the broken-up instructions in the photo. The adverb, as I said, refers to the verb. We interpret it as referring to the whole verb phrase press both buttons as the thing that has to be done in a hard manner, but in fact it's really the pressing that is to be hard. The whole phrase involves the three levels of recursive Merge (recombining constituents) shown above, giving a final nested structure like this:

[3 [2 press [1 both buttons 1] 2] hard 3]

If we want to make it much simpler, one way of doing that is to remove the recursive part of the operation, and have things combine just once. This means, if you assume that Merge is binary (that things can only combine two at a time and not three or more), that the maximum number of words you can have in an utterance is two. And that's exactly what is happening in the photo: two pairs of words (both buttons; press hard), and their juxtaposition is what tells us that one applies to the other rather than their syntactic combination.