anchor.diff:        // the dependency is fine as-is. For example, if
         //
         // * a [0, 1) is disallowed (disallowed)
  -      // * a [1, 2) depends on b [0, 1) (depenency)
  +      // * a [1, 2) depends on b [0, 1) (dependency)
         //
         // there are no changes to be made.
         return dependency;
  
positive.code:       // disallowed entirely. For example, if
        //
        // * a [0, 1) is disallowed (disallowed)
        // * b [0, 1) depends on a [0, 1) (depenency)
        //
        // we can throw away [dependency] and add
        //
  
negative.code:       // the dependency is irrelevant and can be discarded. For example, if
        //
        // * a [0, 2) is disallowed (disallowed)
        // * a [0, 1) depends on b [0, 1) (dependency)
        //
        // we can throw away [dependency].
        return null;
  