Add some comments, rearrange a few things
Handle repeated `define's of the same symbol
parser: check against buffer overflows
fix () evaluation again
(eq? '() '()) => #t (r6rs.pdf, page 39)
Support #t, #f as boolean literals
Make `not' follow R6RS's semantics for it
(list) => ()
Properly print cells with empty-list cdr
prefix: avoid infinite-loop car/cdr self-definition
Tail-recurse on the last element of a `begin' sequence
Fix slight mis-programming in `cons-stream' and `force'
Fix printing of cons/lists containing closures or lists as elements