private Size2 MeasureSubspans(BuildingContext context, SubList <Subspan> subspans)
{
// todo: cache measurments for a context
// todo: consider directions
subspans = TrimEnd(subspans);
var sizes = subspans.Select(MeasureSubspan).ToArray();
return(new Size2(sizes.Sum(x => x.Width), sizes.Max(x => x.Height)));
}
