Compiling:
  public class Package : Object {
      public IndexFile[] indexfiles;

      public void merge(Package other) {
          if (!(other.indexfiles[0] in this.indexfiles))
              this.indexfiles += other.indexfiles[0];
      }
  }

  public class IndexFile : Object { }
  public static void main() { }

Results in:
  mycache.c: In function ‘package_merge’:
  mycache.c:107: error: ‘Package’ has no member named ‘_indexfiles_size_’
  error: cc exited with status 256
  Compilation failed: 1 error(s), 0 warning(s)

If I change the member to 'internal', everything works.