Better definition of `amount(collection)`?

[tkf]

Currently, amount is defined as

SplittablesBase.jl/src/amount.md

Lines 28 to 31 in e87e5b9

	(1) Any empty collection must have zero `amount`.
	(2) Any operation that increments `length` on collection must
	increments `amount`.

So it's an upper bound of length that is monotonically increasing. Maybe this is too strict.

For example, maybe it's better to use the commented out code in

SplittablesBase.jl/src/implementations.jl

Lines 199 to 207 in e87e5b9

	function halve(xs::Iterators.Flatten)
	coll, = arguments(xs)
	left, right = halve(coll)
	return Iterators.flatten(left), Iterators.flatten(right)
	# return _flatten(left), _flatten(right)
	end
	# Is this better?
	# _flatten(xs) = safelength(xs) == 1 ? first(xs) : Iterators.flatten(xs)

which is impossible with the current definition of amount. But the question is how to define a useful property.