anchor.diff:  
         | Block xs
         | Array xs -> 
  -        for x in xs do f x
  +        for x in xs do 
  +          f x
   
         | ForIn(_, a, b, c)
         | Ternary(a, b, c) -> 
  
positive.code:       | Comma(left, right) -> Comma(f left, f right)
        | Block trees -> Block [for t in trees -> f t]
        | Var tree -> Var (f tree)
  
    module Analyzers = 
  
  
negative.code:         f c
  
        | Object xs -> 
          for _, x in xs do
            f x
  
        | Switch(t, xs) -> 
          f t
  