`find` is a little broken, IMO, and we have work to do with similar operators.

The current implementation of find is basically first(fn, undefined). It seems like if you don't "find" the item, it should just complete. Otherwise, I'm not sure what differentiates this operate from first and we can probably remove it.

I understand that the current behavior was meant to model Array.prototype.find, but that's a bit different, because it returns T and not T[]... where our find on Observable<T> returns Observable<T | undefined>, which doesn't make sense to me. What if we're literally trying to find undefined? How do I know I found it?

of(1, 2, 3, 4, undefined, 5, 6, 7).pipe(find(x => x === undefined));

I think our options are:

1. Deprecate and remove find and show people first(fn, undefined) instead.
2. Fix find so that it returns EMPTY if it doesn't "find" anything. Which is a breaking change for some folks for sure.

I'm completely on the fence with this. And maybe we just punt until version 8.

Start by reviewing the current find operator and its relationship to first(fn, undefined), then run through the provided example that searches for undefined. The work is complete only after the project decides between removing or deprecating find and changing it to complete with EMPTY, with the resulting Observable behavior clearly defined.