is a matrix with the preference order of each individual in the market. This argument is only required when utils is not provided. If there are n individuals, then this matrix will be of dimension n-1 by n. The i,jth element refers to j's ith most favorite partner. Preference orders can either be specified using R-indexing (starting at 1) or C++ indexing (starting at 0). The matrix pref must be of dimension n-1 by n. Otherwise, the function will throw an error.