We know that "Not all lemons are yellow", as it has been assumed to be true. We know that "All lemons are yellow", as it has been assumed to be true. Therefore, the two-part statement "All lemons are yellow OR unicorns exist" must also be true, since the first part "All lemons are yellow" of the two-part statement is true (as this has been assumed). However, since we know that "Not all lemons are yellow" (as this has been assumed), the first part is false, and hence the second part must be true.