[plt-scheme] Let, cond and local

As a minor addition to Joe's note, we often use square brackets around
cond lines to help with this particular problem. So, I might have
written this:

(cond
  [(<= 1800 j 1899) 23]
  [(<= 1900 j 2199) 24]
  [else (error "falsches Jahr")])

(newlines are also helpful, since DrScheme's auto indenting also helps
you identify mis-parenthesized text)

Robby