HW11 (due M Dec 9)

Redo Quiz 2 (due at the lecture).

TAPL: 15.2.{1-5}, 15.3.{1,6}, 15.5.3

Extra question: 
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.