'Returns the base form of the given inflected verb, using a rule-based approach.'

def find_lemma(self, verb):

    v = verb.lower()
    if v.endswith(('er', 'ir', 're')):
        return v
    for (a, b) in verb_inflections:
        if v.endswith(a):
            return (v[:(- len(a))] + b)
    return v
