def find_most_frequent(text, sep='.,:;!?- '):
    a = {}
    s = ''
    for i in text.lower() + '.':
        if i not in sep:
            s += i
        elif s:
            a[s] = a.setdefault(s,0) + 1
            s = ''
    if not a:
        return []
    m = max(a.values())
    return sorted([i for i in a if a[i] == m])