On the other hand, in order to induce spatial proximity in the grid to mimic the sequential flow of discourse in documents, we constrain the transition matrix P (which specifies transition preferences from one position in the grid to another) to a sparse banded matrix.
a position i = (i x , i y , i z ) in the grid can only transition to itself, its 4 neighbors in the same XY plane, and two cells in the succeeding two layers along the Z-axis ( .