[Gmsh] Creating a Line Loop from a list of lines (i.e External_Loop[])?

[trophime christophe]

For some reasons I would like to create a Line Loop
from a list of already extisting lines which numbers
are created in a gmsh geometry file using the scripting
langage. I have tried the following according to what I 
have seen in one of the tutorial's examples (the one with
the holes):

Ext_Insert = newreg; Line Loop(Ext_Insert) = {External_Loop[]};

where External_Loop[] is a built-in list of continous lines.
It doesn't work. Gmsh fails with :

Fatal Error : Unknown Line Loop 2

Is there a work-around or Do I miss something?

NB: My External_Loop[] starts at External_Loop[1]