Goal:
    A : Type  B : Type  n : nat  f : A → B  l : list A
  ============================
   (#|map f l| - n = #|l| - n)


DEBUG:Going to execute:
len
DEBUG LTAC Evaluated term: length_skipn
DEBUG LTAC Evaluated term: length_skipn
DEBUG LTAC Evaluated term: length_skipn
DEBUG LTAC Evaluated term: length_skipn