Python string alignment

align.py

from itertools import accumulate

a = ['a', '30', '-', 'foot', 'und', 'ete', 'cted']
b = ['a', '30-foot', 'undetected']


def alignment(a: "Iterable[str]", b: "Iterable[str]") -> "List[List[int]]":
    if ''.join(a) != ''.join(b):
        raise ValueError("a and b must be fragments of the same string")
    target_subs = accumulate(b)
    next_target = next(target_subs, '')
    results, match = [], []
    for idx, current in enumerate(accumulate(a)):
        match.append(idx)
        if current == next_target:
            results.append(match)
            match = []
            next_target = next(target_subs, '')
        elif len(current) >= len(next_target):
            raise ValueError("a must contain only fragments of b")
    return results


print(alignment(a, b))