Represents all variables in a BDD space. This class works as a factory for BDDs in the following way. One instance of this class works as an array, which can be indexed by different keys to obtain different kind of variables. The simplest is boolean variables, which is obtained by using integers, so if x is an instance of gbdd::Vars, then x is the BDD variable with index 2. The expression x & x can then be used to generate a BDD representing that variable 2 and 3 must be true.