No subject

Context here is the TAG (Context) and the value is in TAG[1] (Len(Y))
Looks to me the Eval line must be then Rstr =3D Eval(tag[1],Tag)

I'm afraid you must put the value of Mglobal.U_nom into Tag[1] instead the =
variable name.

Or with luck next can do the job.
  DIM Context AS New Collection
  Context["X"] =3D Tag[0] 'li
  Context["Y"] =3D Tag[1] 'value of Mglobal.U_nom or "Mglobal.U_nom"
  PRINT Eval("Len(Y)", Context)

Just my 2 cents
Ron