How to reference current node in XPath Value rules

Is it possible to reference a current row (and it’s attributes) from within ValueRule XPath expression (Ruleset)?

Simpliest illustrationve use case:
Imagine a datastructure with one entity (ENTITY1) and column COL1 and COL2. I would like to fill COL2 from the value of COL1, so I would expect XPath (of Value Rule with target field COL2) would look like something:

@COL1

or

current()/@COL1

or

/row/@COL1

or

/ROOT/ENTITY1[@Id=current()/@Id]/@COL1

(that’s what I was trying, trying to guess if there is some context of current row)

This use case can be of course solved by using computed function entity field, but I need this for more complicating computations involving selecting (by XPath) from parallel entities.

The XPath rule itself has to have IsPathRelative=True. This way you can directly reference @attr name. Otherwise you have to use full path, e.g. /ROOT/Invoice/InvoiceDetail/@myattr.