ln dropeveryother {
  boolean b;

  io(L in, L out) {
    if (!in.isEmpty())
      b = in[0] == out[0];
  }

  i(L in) {
    new L out;
    for (int i = b ? 0 : 1; i < #in; i += 2)
      out.add(in[i]);
    ret out;
  }
}

Typed on the tablet - that's why it's so short!

But why not always program this way? We just need an expander (translator).