PPT Slide
( length L ) returns a count of the number of top-level elements of L.
( length ’( ( a b ) ( c d ) ) ) 2
( reverse L ) returns a list with the same elements as L, but in reverse order.
( reverse ’( a b c ) ) ( C B A )
( reverse ’( ( a b ) ( c d ) ) ) ( ( C D ) ( A B ))