It provides the int index() method for collections to return the current index of the sequence on them.
It extends ISeq, Sequential, Counted interfaces and thus IPersistentCollection and Seqable too.
One can easily create a wrapper class that gives an indexed functionality to any collection. The following example does just that.
The function shown can be used to wrap sequencial objects in order to keep track of the current index during iteration. This is a simple solution that comes handy when you would need to use map-indexed otherwise.