def METHOD_NAME(self,part,total=1):
    """ compute the number of propa in common
        return the total number of propagator before the two particle (default)
        in addtion it can give the number of uncorelated propagator for the two particle (put total=0) 
    """
    not_defined_mother=[]
    if part.mother in self.all_mother():
        if total:
            return self.level
        else:
            return self.level, self.level-part.level,0
    if self.mother in part.all_mother():
        if total:
            return part.level        
        else:
            print(part.level,self.level)
            return part.level, 0, part.level-self.level
    #=======================================================================
    # try:
    #    if part.mother in self.all_mother():
    #        return self.level
    # except: #self.all_mother not defined
    #    not_defined_mother.append(self)
    #    
    # try:
    #    if self.mother in part.all_mother():
    #        return part.level
    # except: #part.all_mother not defined
    #    not_defined_mother.append(part)
    #    
    # #define all mother if needed
    # for ext_part in not_defined_mother:
    #    ext_part.def_all_mother()
    #=======================================================================
    common=0
    for propa in self.all_mother():
        if propa in part.all_mother() and propa.channel[0]=='S':
            common+=1