[Gmsh] split a surface?

[Martyn]

Hello,

I'm wondering if it is possible to split a surface or add parts of a 
surface to a physical surface.
To put this into context imagine I'm meshing a 'T' shape and I have 
created it from two rectangles; one vertical, the other horizontal. Now 
imagine I wish to define a physical surface which covers the bottom part 
of the horizontal rectangle but not the part which joins the vertical 
part. That's pretty much the situation I'm in.
Ideally I'd liked to do something like:
Physical Surface(Nr) = {HorSurfNr}-{VerSurfNr}

Or split the surface, which ever is possible/easier.

Kind regards,
Martyn