I've played with this a little bit and have a few guesses. My first is that the code was copy-pasta from Snippets, where a matching signature is used for code completion. To look at this yourself put a simple statement somewhere in code, select it, contextual popup menu > create code snippet. Edit the completion (for example, to snozzleberry). Then type snozzleberry in code and it will automatically update itself to the original statement. In such a case, the matching signature is the custom completion. My second guess is that it should work exactly like snippets do but for modifiers, where base is used for in-place substitutions but it hasn't been finished yet. If so, you can have your matchingSignature be snozzleberry, and if your code is "base.setWidth(2 * base.size)", then it would modify with the two base substitutions.