var sequence = { prev: 0, curr: 0 };

function handlePageAdvancement( newUrl ){
  sequence = { prev: sequence.curr, curr: sequence.curr+1 };
  history.pushState( { sequence: sequence }, null, newUrl );
}

$(window).on( 'popstate', function(){
  var _to   = history.state ? history.state.sequence : false;
  var _from = sequence;
  if( !_to || _from.prev === _to.curr ) handleBackwards();
  if( _from.curr === _to.prev ) handleForwards();
  sequence = to;
} );