int a_mathops::momod_echo::run(int din)
{ // First, read a value out of the delay buffer at a point.
  // in the past proportional to setting 'pram1'.
  int delay = w->pram1 * ECHO_TRACE_LEN/100;
  int p = ptr - delay - 1;
  while (p < 0) p+= ECHO_TRACE_LEN;

  // Multiply the old value by 'pram1' to adjust the amount of echo.
  int echo = delaybuffer[p] * w->pram0 / 100;

  // Combine current sample and the echo.
  int r =  din + echo;
  // Write the result into the buffer, so echos re-circulate.
  delaybuffer[ptr] = r;
  ptr += 1;
  if (ptr >= ECHO_TRACE_LEN) ptr = 0;

  return r;
}