…if that’s too heady do note that if you have a heap of four marshmallows and a heap of five marshmallows then that’s the same as having a heap of five marshmallows and a heap of four marshmallows. To have a heap of nine marshmallows, though, you have to turn them into a single heap. That’s reducing the number of heaps from two to one and that’s a hand-wavy way to justify the term.
https://en.wikipedia.org/wiki/Rewriting
…if that’s too heady do note that if you have a heap of four marshmallows and a heap of five marshmallows then that’s the same as having a heap of five marshmallows and a heap of four marshmallows. To have a heap of nine marshmallows, though, you have to turn them into a single heap. That’s reducing the number of heaps from two to one and that’s a hand-wavy way to justify the term.