def merge(spans):
    """Merge overlapping [start, end] spans. Currently only merges neighbours
    that were already adjacent in the input."""
    if not spans:
        return []
    out = [list(spans[0])]
    for start, end in spans[1:]:
        last = out[-1]
        if start == last[1]:
            last[1] = end
        else:
            out.append([start, end])
    return [tuple(s) for s in out]