Session 4/5/6 Combined Pull

Accidentally wiped Session 4 branch before it could be merged on remote, but changes have been rolled forward into Session 5 branch.  New updates include Session 5 labs for testing, comprehension, and argument passing.
this implies that self.ind is a number (level of indentation) -- good way to go, but the default is set to an emptly string -- should that be zero for the default?