The h mapping. The notations X, Y, and Q, refer to the positions (i1, i2, i3), (j1, j2, j3) and (q1, q2, q3) which are illustrated in the figure, respectively. (a) A position Q ∈ (X, Y) is mapped to an index q ∈ (i, j), where i = h(X) and j = h(Y). (b) Some indices q ∈ (i, j) correspond to positions Q = h-1(q) such that Q ∉ (X, Y). This implies that either SX, Qor SQ, Yare not valid sub-instances of S.