def ReverseTraverse(strand, startBase):
    """
    Traverse scaffold/staple strand by a single base in reverse.
    Returns previous adjacent base and block.
    """

    currentBase = startBase
    currentBlock = strand[currentBase[0]][currentBase[1]]

    prevBase = [currentBlock[0], currentBlock[1]]
    prevBlock = strand[prevBase[0]][prevBase[1]]

    return prevBase, prevBlock