A single MDD over variables y1,...,yn cannot be used to represent f, because an MDD can only represent binary-valued functions, not multi-valued functions. Instead, to represent f, we use an array of MDDs, of length equal to the cardinality of Yn+1. Each MDD of this array is defined over y1,...,yn. Furthermore, the minterms for which the ith MDD, fi, evaluates to one, are exactly those minterms for which f evaluates to the ith member of Yn+1. If f is deterministic, then the intersection of fi and fj, for i not equal to j, is empty. If f is completely specified, then the union of the fi's is the tautology. The union of the fi's is referred to as the "domain" of the function.