static void thoughtSpaceDone() {
  AbstractThoughtSpace ts = currentThoughtSpace();
  if (ts instanceof ActualThoughtSpace)
    thoughtSpace(ts.parent);
  else
    print("thoughtSpaceDone without thought space");
}

// set for current thread
static <A extends AbstractThoughtSpace> A thoughtSpace(A ts) {
  currentThoughtSpace_value.set(ts);
  ret ts;
}