Hi there!

This commit adds internal definitions to derived conditional forms, with-fluids and and-let*. This means the bodies of when, unless and with-fluids, and the clause bodies of case and cond behave like a lambda body. 

There is no performance hit since guile optimizes a (let () ...) without internal definitions to a begin (i.e: no new lexical context is created).

best regards
  Linus Björnstam