APL+Win has something it calls a PolyFillMode which Dyalog doesn't seem to have. It determines how interiors of polygons are filled. It has two settings: Alternate & Winding. Dyalog does not support this option. Alternate fills areas that can be reached from outside the figure by crossing and odd number of lines; Winding fills all interior areas. (I haven't checked to see which Dyalog does by default, but I'll eventually need to know what to document, at least.)
APL+Win has something it calls a PolyFillMode which Dyalog doesn't seem to have. It determines how interiors of polygons are filled. It has two settings: Alternate & Winding. Dyalog does not support this option. Alternate fills areas that can be reached from outside the figure by crossing and odd number of lines; Winding fills all interior areas. (I haven't checked to see which Dyalog does by default, but I'll eventually need to know what to document, at least.)