Do not use Function.prototype.bind on it.only

designs/2020-rule-tester-only/README.md

@@ -46,8 +46,8 @@ To allow using `RuleTester` with a custom test framework other than Mocha, paral
 1. Add an `itOnly` `set` accessor that sets `RuleTester[IT_ONLY]`.
 1. Add an `itOnly` `get` accessor.
    1. If `RuleTester[IT_ONLY]` is set, return it.
-   2. If global `it` and `it.only` are functions, return `Function.bind.call(it.only, it)`.
-   3. Throw an error:
+   2. If global `it` and `it.only` are functions, return `it.only`. *Note: Do not return `Function.bind.call(it.only, it)` because this does not work with Mocha. If `this` is important, we could return a wrapper function.*
+   1. Throw an error:
       1. If either `RuleTester[DESCRIBE]` or `RuleTester[IT]` is customized, recommend setting a custom `RuleTester.itOnly`.
       2. If global `it` is a function, the current test framework does not support `only`.
       3. Otherwise recommend installing a test framework like Mocha so that `only` can be used.