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.

𝓑𝓻𝓸𝓽𝓱𝓮𝓻 𝓘 𝓻𝓮𝓬𝓮𝓲𝓿𝓮𝓭 𝔂𝓸𝓾𝓻 𝓶𝓮𝓼𝓼𝓪𝓰𝓮 𝔀𝓮𝓵𝓵. 𝓘𝓽 𝓮𝔁𝓬𝓲𝓽𝓮𝓼 𝓶𝓮 𝓽𝓸 𝓲𝓷𝓯𝓸𝓻𝓶 𝔂𝓸𝓾 𝓪𝓫𝓸𝓾𝓽 𝓪 𝓬𝓸𝓬𝓴 𝓻𝓮𝓼𝓮𝓻𝓿𝓮, 𝓯𝓻𝓮𝓽 𝓷𝓸𝓽, 𝓬𝓸𝓬𝓴 𝔀𝓲𝓵𝓵 𝓫𝓮 𝓼𝓮𝓷𝓽 𝔂𝓸𝓾𝓻 𝔀𝓪𝔂 𝓼𝓸𝓸𝓷.