public class InterpretException extends SLException {
  private String msg;
  private final String top = "\n[top level]";
  private String callStack="\nCall stack:";
  
  public InterpretException(String s) { msg="Interpreter error: "+s; }
  public void addToCallStack(String fun) { callStack += "\n"+fun; }
  public String getCallStack() { return callStack+top; }
  public String getMessage() { return msg + callStack+top; }
}