function jump(history, n) {
    if (n > 0) return jumpToFuture(history, n - 1);
    if (n < 0) return jumpToPast(history, history.past.length + n);
    return history;
}
function jump(history, n) {
    if (n > 0) return jumpToFuture(history, n - 1);
    if (n < 0) return jumpToPast(history, history.past.length + n);
    return history;
}
function jump ( history , n ) { if ( n > 0 ) return jumpToFuture ( history , n - 1 ) ; if ( n < 0 ) return jumpToPast ( history , history . past . length + n ) ; return history ; }
jump: jump n steps in the past or forward
jump : jump n steps in the past or forward