nvlColValue ::
        ColumnName  -- ^ ColumnName key
    ->  RDataType   -- ^ value returned if original value is Null
    ->  RTuple      -- ^ input RTuple
    ->  RDataType   -- ^ output value
nvlColValue col defaultVal tup = 
    if isRTupEmpty tup
        then Null
        else 
            case tup <!> col of
                Null   -> defaultVal
                val    -> val 
nvlColValue ::
        ColumnName  -- ^ ColumnName key
    ->  RDataType   -- ^ value returned if original value is Null
    ->  RTuple      -- ^ input RTuple
    ->  RDataType
nvlColValue col defaultVal tup = 
    if isRTupEmpty tup
        then Null
        else 
            case tup <!> col of
                Null   -> defaultVal
                val    -> val 
nvlColValue col defaultVal tup = 
    if isRTupEmpty tup
        then Null
        else 
            case tup <!> col of
                Null   -> defaultVal
                val    -> val 