abstract sclass PStackComputableWithStep<A> extends PStackComputable<A> {
  int step;
  
  // phasing out the signature with subComputationResult
  public final void step(VStack stack, O subComputationResult) {
    step((IPStack) stack);
  }
  
  void step(IPStack stack) {}
  
  toString { ret shortClassName(this) + " step " + step; }
}