(len coll)
  coll: a deque, a tab, or an iter
  returns an int or a sym

Returns the number of items in a collection.

When the argument is a table, string or array, it returns the number of elements or entries stored by that collection.

The argument may also be an iterator. In that case, the result is:

Iterators are quite conservative when reporting their length. For example, array iterators report their length as unknown, because it's possible that additional elements may be pushed to the array during iteration.