Hi Peter,

vars MB = series((priceHigh(0)+priceLow(0))/2);

This calculates the series of the mid-points of the bars.

Zorro only runs at the start of a new bar, so in your example the midpoint of last bar(the one immediately closed) is MB[0]. There is no data yet in the current open bar.

MB[1] is then 2nd last closed bar etc.