nnot slide outside the volume range 0..64. The
     period-length in this channel's division is a parameter to this
     effect, and hence is not played.

[6]: Continue 'Vibrato', but also do Volume slide
     Where [6][x][y] means "either slide the volume up x*(ticks - 1) or
     slide the volume down y*(ticks - 1), at the same time as continuing
     the last 'Vibrato'". It is illegal for both x and y to be non-zero.
     You cannot slide outside the volume range 0..64.

[7]: Tremolo
     Where [7][x][y] 