human
I wrote the below code

    assert isinstance(spec_str, string_types)
    tokens = re.findall(VSPEC_TOKENS, (u'(%s)' % spec_str))
    output = []
    stack = []
    def apply_ops(cstop):
        while (stack and (stack[(-1)] not in cstop)):
            if (len(output) < 2):
                raise InvalidVersionSpecError(spec_str)
            c = stack.pop()
            r = output.pop()
            r = (r[1:] if (r[0] == c) else (r,))
            l = output.pop()
            l = (l[1:] if (l[0] == c) else (l,))
            output.append((((c,) + l) + r))
    for item in tokens:
        item = item.strip()
        if (item == u'|'):
            apply_ops(u'(')
            stack.append(u'|')
        elif (item == u','):
            apply_ops(u'|(')
            stack.append(u',')
        elif (item == u'('):
            stack.append(u'(')
        elif (item == u')'):
            apply_ops(u'(')
            if ((not stack) or (stack[(-1)] != u'(')):
                raise InvalidVersionSpecError(spec_str)
            stack.pop()
        else:
            output.append(item)
    if stack:
        raise InvalidVersionSpecError(spec_str)
    return output[0]

What's a good function header?
 ... Here's a good function header?
assistant
#Here's a good function header.

def treeify(spec_str):