def _constraints(self, X, missing_mask, S, error_tolerance):
        """
        Parameters
        ----------
        X : np.array
            Data matrix with missing values filled in

        missing_mask : np.array
            Boolean array indicating where missing values were

        S : cvxpy.Variable
            Representation of solution variable
        """
        ok_mask = ~missing_mask
        masked_X = cvxpy.multiply(ok_mask, X)
        masked_S = cvxpy.multiply(ok_mask, S)
        abs_diff = cvxpy.abs(masked_S - masked_X)
        close_to_data = abs_diff <= error_tolerance
        constraints = [close_to_data]
        if self.require_symmetric_solution:
            constraints.append(S == S.T)

        if self.min_value is not None:
            constraints.append(S >= self.min_value)

        if self.max_value is not None:
            constraints.append(S <= self.max_value)

        return constraints
Parameters
        ----------
        X : np.array
            Data matrix with missing values filled in

        missing_mask : np.array
            Boolean array indicating where missing values were

        S : cvxpy.Variable
            Representation of solution variable