Error thrown on simple equation

[victorBaro]

Thanks for this library, is working really well in many cases.

I am facing an issue though with a specific case:

        let solver: Solver = Solver()
        
        let row0 = Variable("row_0")
        let row1 = Variable("row_1")
        let row2 = Variable("row_2")
        let row3 = Variable("row_3")

        do {
            try solver.add(constraint: row0 == row1)
            try solver.add(constraint: row2 == row3)
            try solver.add(constraint: row0 + row1 + row2 + row3 == 240)
            try solver.add(constraint: row0 + row1 == 120)
            try solver.add(constraint: row2 + row3 == 120)
        } catch {
            throw error
        }

This simple equation is raising an error: caught error: "unable to satisfy constraint: 1.0 * row_2 + 1.0 * row_3 + -120.0 == 0

If I set any of the last 3 constraints as .strong (instead of required), the solver is able to resolve (all variable values are 60).