anchor.diff:    }
   
     @override
  -  Option<hir.Expression> visitIsExpression(IsExpression node) {
  +  Option<hir.Expression> visitIsExpression(hir.IsExpression node) {
       if (node.id == id) return Some(node);
       return node.instance.accept(this);
     }
  
positive.code:     if (loweredThenBody is Error) return loweredThenBody;
      final thenBody = loweredThenBody.value;
  
      final elseBody = <Expression>[];
      if (theIf.elseKeyword != null) {
        assert(theIf.elseBody != null);
        final elseContext = IfContext(this, getId(theIf), None());
  
negative.code:   }
  
    @override
    List<hir.Identifier> resolveIdentifier(String name) {
      final result = _identifiers[name];
      if (result != null) return [result];
      return parent.value.resolveIdentifier(name);
  