[plt-scheme] abstract wall of recursion

Problem 14.3.4 and 15.1.2 were certainly challenging but i was able to
solve them by not thinking too deeply about the
recursive procedures embedded within both solutions. And
that is the problem if i try to think too much about what is
going on, i feel i'm hitting an abstract wall made of steel.
 I hope this unsettled feeling is part of the learning process
and will pass.
mike