2i is one token — the lexer's NUMBER ends 'i'? — so a written 2i arrives as a single number and not as a product with anything to rename. Under a binder that names i it is nevertheless the writer's 2 beside the writer's i, and 2e is a product there, so this reads it as one: sum(2i, i, 1, 3) is 12 rather than 6i.