'"Left as an exercise to the reader" (LAAETTR) are perhaps the most hated 7 words in all of mathematics. Every student, whether dying to just finish a problem set for a required class or genuinely trying to attain a robust understanding of a new concept, has encountered these infamous words. They represent the worst education has to offer - the tantalizing promise of understanding, swiftly obliterated. And worse, this obliteration is nominally done for the sake of the student. Rigor! The process of learning! You need to figure it out! Nonsense.