Substring tests with similar intent but different expected output

There are two tests in CqlStringOperatorsTest.xml that seemingly test the same concept (Substring of a string, starting at string.length) but have different results, empty string vs null.

<test name="SubstringAB2" version="1.0">
	<capability code="string-operators" />
	<expression>Substring('ab', 2)</expression>
	<output>null</output>
</test>

<test name="SubstringEmptyAnd0" version="1.0">
	<capability code="string-operators" />
	<expression>Substring('', 0)</expression>
	<output>''</output>
</test>

The most relevant sections of the spec I could find are in the Developer's Guide, String Operators: https://cql.hl7.org/R2/03-developersguide.html#string-operators

The second argument is the starting index of the substring to be returned, and the third argument is the length of the substring to be returned. If the length is greater than the number of characters present in the string from the starting index on, the result includes only the remaining characters. If the starting index is less than 0, or greater than the length of the string, the result is null. The third argument is optional; if it is not provided, the substring is taken from the starting index to the end of the string.

and the CQL Reference: https://cql.hl7.org/09-b-cqlreference.html#substring

If stringToSub or startIndex is null, or startIndex is out of range, the result is null.

(note "out of range" is not defined more specifically anywhere that I saw)

In both these cases, the starting index is not less than zero, nor is it greater than the length of the string. So by that reading I think SubstringEmptyAnd0 is correct and the expected output for both should be empty string. That test was added more recently, with some discussion, here: https://github.com/cqframework/cql-tests/pull/55

However, I'll also note that in FHIRpath, the substring operator is more clearly defined on this case:

https://hl7.org/fhirpath/en/#substringstart--integer--length--integer--string

If start lies outside, or equal to, the length of the string, the function returns empty ({ }). If there are less remaining characters in the string than indicated by length, the function returns just the remaining characters.

with the specific example

''.substring(0) // { } (start position is outside the string)

(CQL is not FHIRPath but as I understand it there's a strong push to keep them aligned)

I don't have a strong sense either way, other than I think these 2 test cases should have the same expected output.

Submitting this as an issue instead of a PR to invite discussion

Start with the two cases in CqlStringOperatorsTest.xml, then read the cited String Operators section of the Developer's Guide and the CQL Reference. Compare their handling of a start index equal to string length, including the linked discussion in pull request 55. Done means the interpretation is resolved and the two tests have consistent expected outputs.