HW11 (due Tuesday Dec 2 at 2pm)

TAPL: 15.2.{1-5}, 15.3.{1,6}

Extra questions (about the Arrow rule):

1. Give an example to demonstrate both the co-variant (output)
   and contra-variant (input) of the S-Arrow rule.

   Try to be as different from the example from the slides as possible.

2. What if we change the S-Arrow rule to the more "symmetric" version:

 S1 <: T1  and  S2 <: T2 
------------------------- S-Arrow'
     S1->S2 <: T1->T2

Show an example term that typechecks but stucks in the evaluation process.