CFL Closure, Decisions and Pumping
Closure of CFLs (union, concatenation, star, reversal, homomorphism, inverse homomorphism, intersection with a regular language) and the failures (intersection, complement, difference) with witnesses; decidable and undecidable questions for CFGs; the pumping lemma for CFLs with the vxy window argument and the strings to choose; closure proofs of non-context-freeness; Parikh's theorem for unary languages; and the standard non-CFL languages with the complement trap. Grammar and PDA claims were checked by script.