Replaced append method with +.
Merge branch 'master' of github.com:/brownplt/pyret-lang
Merge branch 'master' of github.com:/brownplt/pyret-lang
Merge branch 'master' of github.com:/brownplt/pyret-lang
Merge branch 'master' of github.com:/brownplt/pyret-lang
Merge branch 'master' of github.com:/brownplt/pyret-lang
Merge branch 'master' of github.com:/brownplt/pyret-lang
Merge branch 'master' of github.com:/brownplt/pyret-lang
added index (list-ref) to list object