=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
}}}

{{{
def remove_diag(diag):
    """remove all diagram which are not pure QCD or pure QED: Fake example"""

    if diag.get('orders')['QCD'] >0 and diag.get('orders')['QED']>0:
        return True

    return False
}}}

{{{
import madgraph.various.misc as misc

def remove_diag(diag):
    """force to have two s-channel W decaying into a specific way. 
       Designed for interference studies where decay chain are not allowed
       This is designed for keeping the following diagram generate p p > j j w- w-, w- > e- ve~ 
       from the following syntax:  p p > j j e- ve~ e- ve~
"""

    found_w = 0
    for vertex in diag['vertices']:
        #misc.sprint(vertex.nice_string())
        if vertex.get('id') == 0: #special final vertex
             continue
        if len(vertex['legs'])!=3:
            continue

        for i, leg in enumerate(list(vertex['legs'])):
            #misc.sprint(i, leg)
            if abs(leg['id']) != 24:
                continue
            else:
                break
        else:
            continue
        # we have a vertex with a W
        if i == 2: #This means a vertex A B > W or W > A B
            if leg['number'] > 2: # means that we have W > A B
                #misc.sprint(vertex.nice_string())
                # check that A B are what we want
                if not all(oleg['state'] for oleg in vertex['legs'][:2]) : # this means not final state
                    continue
                #misc.sprint([oleg['id'] for oleg in vertex['legs'][:2] ])
                if set([oleg['id'] for oleg in vertex['legs'][:2] ]) == set([11,-12]):
                    found_w +=1
        else: # this means A W > B
            if vertex['legs'][-1].get('number') < 3:
                # T-channel -> continue
                continue
            else:
                #check that they are both final state
                if not vertex['legs'][-1]['state']:
                    continue
                if not vertex['legs'][1 if i==0 else 0]['state']:
                    continue

                other_id = [vertex['legs'][-1].get('id'), vertex['legs'][1 if i==0 else 0]['id']]
                if set(other_id) == set([11,-12]):
                    found_w += 1

    misc.sprint(found_w)
    if found_w == 2:
        return True
    else:
        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.
}}}