OP annotation of expression that evaluates voltages of terminals' nets.

Hello,

I have faced some practical problem, when it is need for schematic subcircuit to evaluate voltage terminal difference like G-S and annotate it as OP point parameter.
With respect to existing documentation expression for opParamExprList should look like ("vgs" "VDC(root(\"G\"))-VDC(root(\"S\"))"), where G and S are terminal names of subcircuit.
However, nets instance/G and instance/S are resolved to net names of instance instantiation in top schematic and voltages are not save for by such names.

Is there any SKILL function (that can be used for opParamExprList ) to resolve subcircuit nets like instance/G and instance/S to their actual names for voltage accessing?!
Note: Trick with creating internal nodes by some extra series components is known, works, but not appreciated. The same about construction of expression with OP()s.

BR Denis