Commutative operations (all of them I think?) are trivially generalized to n-ary operations (in fact, we do this via ∑ and ∏, in the case of addition and multiplication, respectively). You're right that the question of what "operation" we're dealing with here is a bit hazy; but I'd wager that it's probably in the family of the increment operation (N++ === N + 1 = 1 + N) since we're constantly evaluating the next line of code, like the head of a Turing machine.
Edit: maybe it's actually implication? Since the previous line(s) logically imply the next. L_0 → L_1 → L_2 → L_n? Though this is non-commutative. Not sure, it's been a few years since my last metalogic class :P
Implication sounds right. With no further analysis, running each line in order is correct (for whatever "order" is defined by a language, let's assume imperative).
A compiler could recognise that e.g. L_2 doesn't depend on L_1, and would be free to reorder them. And compilers do recognise this in terms of data dependence of operations.
Generalizing an associative binary op to an n-ary op just requires an identity element Id (which isn't always obvious, e.g. Id_AND=true but Id_OR=false).
> Generalizing an associative binary op to an n-ary op just requires an identity element Id (which isn't always obvious, e.g. Id_AND=true but Id_OR=false).
Only for n = 0, I think. Otherwise, generalizing associative binary f_2 to f_n for all positive integers n is easily done inductively by f_1(x) = x and f_{n + 1}(x_1, ..., x_n, x_{n + 1}) = f_2(f_n(x_1, ..., x_n), x_{n + 1}), with no need to refer to an identity. (In fact, the definition makes sense even if f_2 isn't associative, but is probably less useful because of the arbitrary choice to "bracket to the left.")
Edit: maybe it's actually implication? Since the previous line(s) logically imply the next. L_0 → L_1 → L_2 → L_n? Though this is non-commutative. Not sure, it's been a few years since my last metalogic class :P