The FIRST manual and Dave Lavery both mention to look more at the intention of the rules and not so much the nit picky details. From that perspective, my interpretation is that the game designers wanted a single 2X ball to cap a goal. The 30" ball is larger than the 28" diameter of all the goals, and would normally allow for only 1 large ball to be placed on top.
"CAPPING - A LARGE ball is considered to be CAPPING a goal if it is SUPPORTED by the goal poles and/or SCORED SMALL Balls. ..."
This part of the definition would stand to reason that a large ball touching another large ball would not count.
Yeah, yeah, I know that a large ball could be stuffed in the goal, several more small balls placed above it, and then a second large ball could be placed on top. Technically this second ball isn't touching the first large ball stuffed in the goal, but it certainly is not what the game planners had in mind.
Common sense approach should help in determining this year's game and objectives. At least this year we don't have to deal with "What constitutes a STACK?"
