[NTG-context] Spacing within combinations

Greetings, The attached MWE combines 12 images in a 3 by 4 grid; with horizontal and vertical spacing set by setupcombinations. I have two questions. First, can someone please tell me why the vertical spacing is not applied between the first and second and third and fourth rows? Second, why, If I try, e.g.c 15 images, does ConTeXt complain of a missing left brace. More generally, my mwe failed with an odd number of images, but succeeded with an even number. Thanks. With kind regards, Michael http://literatesolutions.com