Return a tabbed string from an expanded one.

def re_tab(s):
    """Return a tabbed string from an expanded one."""
    l = []
    p = 0
    for i in range(8, len(s), 8):
        if s[i - 2:i] == "  ":
            # collapse two or more spaces into a tab
            l.append(s[p:i].rstrip() + "\t")
            p = i

    if p == 0:
        return s
    else:
        l.append(s[p:])
        return "".join(l)