Define an index set I = V ∪ E. A derivation is represented by a vector y = {y r : r ∈ I} where y v = 1 if vertex v is used in the derivation, y v = 0 otherwise (similarly y e = 1 if edge e is used in the derivation, y e = 0 otherwise).
y is a vector in {0, 1} |I| .