def gen_unknown_filter(skip_unknown = False):
    """
    Generation of a SPARQL Filter clause to exclude unknown edge type
    Produces something like 
        FILTER( ?controlType = "ACTIVATION"^^xsd:string || ?controlType = "INHIBITION"^^xsd:string)
    """
    if skip_unknown == True:
        return 'FILTER( ?controlType = "ACTIVATION"^^xsd:string || ?controlType = "INHIBITION"^^xsd:string)'
    else:
        return ''