The stack monoid revisited
From Wikipedia:<p>> In abstract algebra, a branch of mathematics, a monoid is a set equipped with an associative binary operation and an identity element.<p>So M = (S, op, id)<p>My go-to example of a monoid is (the set of strings, string concatenation, the empty string). It's my go-to because there's e.g. no `x` such that `concat("blah", x) == ""`. Another example is (integers, +, 0), but it is not my go-to example because it also more than a monoid.<p>I did not find a clear explanation of what the stack monoid is here. Can someone state it, please?