[Gmsh] Problem about 'Plugin Crack'

[LU Xiaoxin]

Dear GMSH users and developers,

I have a question about 'Plugin crack' command.
In the manual, it is said that 'The elements touching the crack on the 
“negative” side are modified to use the newly generated vertices'.
I was wondering how to define the 'negative' side for 2D crack. Is it 
defined by NormalX, Normal Y and Normal Z, which are the components of 
normal vector of the crack surface?
Thank you very much.

Best ragards,
Xiaoxin