Feature request: extend built-in functions

Hello.
We need to add some more built-in functions.

One would be the diff between two numbers. Another function would make a number negative. We can contribute with the code ourselves if it is ok.

Start by locating the JavaScript implementation's built-in function definitions and their tests. Review how existing numeric functions are specified and tested, then define the expected behavior for numeric difference and negation before adding coverage and implementation.

Written by the indexing model from the issue text.