Add example of how index can be used to reverse a list

[krlawrence]

Gremlin does not have a reverse step but a list can be reversed using the index step couples with order and tail

It is possible to do this today using just existing Gremlin steps. The example below takes advantage of the index step to give each element of a list an index number. For example:

    gremlin> g.inject(['A','B','C','D']).index()
    ==>[[A,0],[B,1],[C,2],[D,3]]   

Given that building block, we can use those index values and order the list.

    gremlin>  g.inject(['A','B','C','D']).index().
    ......1>    unfold().
    ......2>    order().
    ......3>      by(tail(local,1),desc)
    ==>[D,3]
    ==>[C,2]
    ==>[B,1]
    ==>[A,0]   

The last step is to return the reordered list with the index values removed.

    gremlin>  g.inject(['A','B','C','D']).index().
    ......1>    unfold().
    ......2>    order().
    ......3>      by(tail(local,1),desc).
    ......4>    limit(local,1).
    ......5>    fold()  
    ==>[D,C,B,A]