A 'sum' is a sequence of terms joined by addition.
A 'product' is a sequence of terms joined by multiplication.
Is there a general term for terms joined by a general associative operation, that mathematicians would know? Is it 'sequence'?
In Haskell this would be implemented as a fold, but what do you call the thing it acts on? 'Iterable' and 'Enumerable' are too computer-sciencey.
It's possible this question has no good answer.

@christianp Induction/recursion principle maybe? Or σ-algebra.

@amiloradovsky you're naming the process, and I want a name for the terms, considered together

@christianp Folding in ML is also an operation (process). I don't think there is a better name for it, or it wouldn't be called "folding".
"Combinator" is yet another possibly-related term.

@christianp Anyway, you can always speak about a sequence of elements in a magma…

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