def like_cdid(xycell):
    """
    Given an xycell return True if the .value attribute of the cell _appears_ to be a CDID code, a CDID code is
    an upper cased alpha numeric code of length four. 
    """
    if isinstance(xycell.value, str) and xycell.value.isalnum() == True and xycell.value.isupper() == True and len(xycell.value) == 4:
        return True
    else:
        return False