GridLineDesign #
The q diagonals of a cluster pair partition its q² block pairs: the map sending a
diagonal label a and a position j to the block pair (j, j + a) is a bijection. This is the
allocation form of Nibble.AX1.gridUW_bijective: a cluster pair shared by several cluster triples
is exactly covered as soon as the diagonals are distributed among them.
Two block sub-triples of a family of lines with distinct first labels never use the same
block pair of the cluster pair (U, W).
Two block sub-triples of a family of lines with distinct second labels never use the same
block pair of the cluster pair (U, X).
Two block sub-triples of a family of lines with distinct differences never use the same
block pair of the cluster pair (W, X).
The allocation criterion. Two cluster triples through the cluster pair (U, W) that are
allocated disjoint sets of diagonals never use a common block pair of that cluster pair: the
rectangles of the two triples inside U × W are disjoint.
The diagonals of one cluster pair are exactly q, and each has exactly q block pairs.
Together with Nibble.AX1.diagIndex_bijective this is the counting behind the allocation: the
diagonals allocated to the cluster triples through a pair tile that pair as soon as they partition
ZMod q.