Recurrence gives incorrect values when the order of previous states is important.

eg
recurrence!"a[n-1] + a[n-2]"(1,2) works, but
recurrence!"a[n-2]"(1,2) does not.

fix:

    void popFront()
    {
        _state[_n % stateSize] = binaryFun!(fun, "a", "n")(
-           cycle(_state, _n), _n + stateSize);
+           cycle(_state), _n + stateSize);
        ++_n;
    }

The old values array is being offset twice, remove the argument to cycle and it works perfectly.