sclass RepeatColumn extends Predictor {
  float[] nextColumn(float[] x) {
    ret x;
  }
}

sclass ShiftDown extends Predictor {
  float[] nextColumn(float[] f) {
    if (f == null) null;
    float[] g = copyColumn(f);
    for (int i = 1; i < l(f); i++)
      g[i] = f[i-1];
    ret g;
  }
}

sclass LookLeft extends Predictor {
  int width; // how far to look left
  float[][] buf;
  int x;
  
  float[] nextColumn(float[] f) {
    if (f == null) null;
    if (buf == null) buf = new float[width][];
    
    // exchange with data in rotating buffer
    // (will return null until buffer is filled)
    float[] g = buf[x];
    buf[x] = f;
    x = (x+1) % width;
    ret g;
  }
}