Optional Assertion

[STRd6]

I recently wanted to do something like:

HashThis
  LengthShorthand:id &( ( _+ ( ( Not __ In ) / In ) ) )?:beforeIn ->
    if (beforeIn) ...

but ended up needing to do:

LengthShorthand:id ( &( _+ ( ( Not __ In ) / In ) ) "" )?:beforeIn ->
  if (beforeIn != null) ...

This is because assertions always return undefined since they fail the parse if not met. This causes issue with ? because it returns undefined if the parse wasn't matched so there's not a simple way find out whether the optional assertion matched or not.

It would be nice if optional assertions translated to true/false for whether the assertion passes or fails.

Rule
  A &B?:b ->
    // b is `true` or `false`