code: function sc_isList(o) {
      var rabbit;
      var turtle;
  
      var rabbit = o;
      var turtle = o;
      while (true) {
  	if (rabbit === null ||
  	    (rabbit instanceof sc_Pair && rabbit.cdr === null))
  	    return true;  // end of list
  	else if ((rabbit instanceof sc_Pair) &&
  		 (rabbit.cdr instanceof sc_Pair)) {
  	    rabbit = rabbit.cdr.cdr;
  	    turtle = turtle.cdr;
  	    if (rabbit === turtle) return false; // cycle
  	} else
  	    return false; // not pair
      }
  }
docs: * META ((export #t)
  (type bool))