xPath like UNL's

Stores the following information within the UNL:

NodeHeader[A][B]

Where:
A = nth of sibling
B = nth of sibling with same name

For clearance, in an outline such as:

a
 b
 c
 c


The last node UNL would be:

c[2][1]

The previous one:
c[1][0]

the previous:
b[0][0]