new file:   11-multirember-letrec.scm
	new file:   12-member-letrec.scm
	new file:   12-multirember-f.scm
	new file:   12-scramble.scm
	new file:   12-sum-of-prefixes-letrec.scm
	new file:   12-to-in-a-row-letrec.scm
	new file:   12-union-letrec.scm