substring: what to return if the start index is outside the first argument string, or the desired length exceeds the remaining string length

-Empty string should be returned for an index larger than the length of the string. -Length too long should be allowed and return as many characters up to the specified length. -The behavior for a negative index is undefined. -A negative length is a bad request. Because negative index is undefined, interoperable clients should not use negative indexes. In 4.01, negative indexes mean count from the end, see ODATA-901