code: def filter_seq(seq):
       '''Examines unreserved sequences to see if they are prone to mutation. This
       currently ignores solely-power-of-2 guides with b > 3'''
       if seq.res:
            return None
       n = nt.Factors(seq.factors)
       guide, s, t = aq.canonical_form(n)
       seq.guide = guide
       cls = aq.get_class(guide=guide)
       num_larges = seq.factors.count('P')
       upper_bound_tau = cls - num_larges - len(t)
       if cls < 2 or upper_bound_tau < 2:
            return None
       v = nt.Factors({p: a for p, a in guide.items() if p != 2 and a > 0})
       if int(v) == 1 and cls > 3:
            return None
       if not aq.is_driver(guide=guide):
            return None
       return n, guide