add support for operator extensions in data-sparql11/functions/plus-[12](-corrected)?

[lisp]

these tests exemplify those which verify the argument value domain constraints codified by the recommendation. in this case, the tests' results indicate that the '+' function accepts numbers only.
there should be some means whereby the tests provide for the extensions allowed by 17.6.

in this case, the extension which is motivated by frequently appearing queries is to perform automatic string-to-number conversion, which yields different solutions.