iterator-helpers.md

Some iterator helpers already limit the reversibility. eg. take(n), actually it means take the first n elements, so it already one-direction and no way to reverse.

Or we could make iterators always reversible (means u could always reverse() it, reverse() just reverse the invoking of next() and nextLast()), because it just change the end which could be iterated.

There are three type of iterables as double-ended iteration:

1. double-ended, which its iterator support both next() and nextLast()
2. forward-only, which its iterator only support next(), reverse() it will transform it to backward-only
3. backward-only, which its iterator only support nextLast(), reverse() it will transform it to forward-only

Here are the simple result table for iterator helpers (note, most aggregator methods like toArray, every, etc. are direction-neutral so not listed here) :

method \ upstream	double-ended	forward-only	backward-only
forEach	✅ double-ended	✅ forward-only	✅ backward-only
map	✅ double-ended	✅ forward-only	✅ backward-only
filter	✅ double-ended	✅ forward-only	✅ backward-only
flatMap	✅ double-ended	✅ forward-only	✅ backward-only
reverse	✅ double-ended	✅ backward-only	✅ forward-only
indexed	✅ forward-only	✅ forward-only	❌
take	✅ forward-only	✅ forward-only	❌
drop	✅ double-ended	✅ forward-only	❌
find	✅	✅	❌
reduce	✅	✅	❌
takeLast	✅ backward-only	❌	✅ backward-only
dropLast	✅ double-ended	❌	✅ backward-only
findLast	✅	❌	✅
reduceRight	✅	❌	✅

Note: How to read the table: upstream.method() => tablecell, for example, doubleEnded.map() => doubleEnded, forwardOnly.reverse() => backwardOnly, forwardOnly.takeLast() => ❌ (❌ means throw TypeError).

Instead of throwing TypeError for ❌ , another option is return a "dead" iterator (which have no next() and nextLast()), though it seems useless.

Maybe take, takeLast could keep double-ended, see #13

About flatMap: need more investigation.