// for stuff like #1000478

static class LIntersperse extends LearnerImpl {
  static boolean debug;
  ReversibleFunction preprocessor; // e.g. JavaTok
  L<S> outPrefix, outSuffix, filler;

  *(ReversibleFunction *preprocessor) {}
  
  public void processInOut(Object _in, Object _out) {
    if (debug)
      debug("i/o " + _in + " " + _out);
    L<S> in = cast preprocessor.process(_in);
    L<S> out = cast preprocessor.process(_out);
    int i = out.indexOf(in.get(1));
    outPrefix = out.subList(0, i);
    int j = out.lastIndexOf(in.get(in.size()-2));
    outSuffix = out.subList(j+1, out.size());
    int newLength = j-i+1;
    int realOldLength = (in.size()-1)/2;
    
    /*int intersperse = 0;
    if (realOldLength > 1)
      intersperse = newLength-realOldLength*3-1;*/
    int intersperse = 3; // just assume this for now
    if (realOldLength > 1)
      filler = out.subList(i+1, i+1+intersperse);
    
    if (debug)
      debug("oldLength= " + in.size() + ", newLength=" + newLength +", realOldLength=" + realOldLength + ", intersperse=" + intersperse + ", filler length=" + (filler == null ? 0 : filler.size()));
  }
  
  public Object processIn(Object _in) {
    L<S> l = cast preprocessor.process(_in);
    if (debug)
      debug("i " + structure(l));
    new L<S> list;
    list.addAll(outPrefix);
    if (debug)
      debug("ranging from " + 1 + " to " + (l.size()-2));
    for (int i = 1; i < l.size()-1; i += 2) {
      list.add(l.get(i));
      if (i < l.size()-2)
        list.addAll(filler);
    }
    list.addAll(outSuffix);
    Object out = preprocessor.unprocess(list);
    if (debug)
      debug("output: " + structure(list) + " = " + structure(out));
    return out;
  }
}

Began life as a copy of #1000468