=How can I remove some unwanted diagram?

== Warning

Be carefull about gauge invariance (a non gauge invariant subset will also break lorentz invariance)

== Requirements:

 1. This method is implemented since MG5aMC version 2.5.0
 1. This method is only implemented for leading order processes

== method:

=== user_filter.py

You need to define the file user_filter.py who should define the function remove_diag(diag):
It should return True if you want to discard the diagram.

example of file
{{{
def remove_diag(diag):
    """remove all diagram with quark in T-channel"""
    for vertex in diag['vertices']: #last
         if vertex['id'] == 0: #special final vertex
             continue
         if vertex['legs'][-1]['number'] < 3: #this means T-channel
             if abs(vertex['legs'][-1]['id']) <6:
                  return True
    return False
}}}

=== running with user_filter

to activate the filtering, you have to add '--diagram_filter'
to the command 
example
{{{
generate p p > w+ w+ j j --diagram_filter
}}}

If you remove any diagram you should see on the screen:
{{{
INFO: Trying process: c c > w+ w+ s s WEIGHTED<=6 @1
WARNING: Diagram filter is ON and removed 12 diagrams for this subprocess.
}}}