```
using JuMP, BilevelJuMP, Xpress
model = BilevelModel()
@variable(Lower(model), x)
@variable(Upper(model), y)
@objective(Upper(model), Min, 3x + y)
@constraints(Upper(model), begin
x <= 5
y <= 8
y >= 0
end)
@objective(Lower(model), Min, -x)
@constraints(Lower(model), begin
x + y <= 8
4x + y >= 8
2x + y <= 13
2x - 7y <= 0
end)
optimize!(model, Xpress.Optimizer(), BilevelJuMP.SOS1Mode())
objective_value(model) # = 3 * (3.5 * 8/15) + 8/15
value(x) # = 3.5 * 8/15
value(y) # = 8/15
```

Run the example in the readme file. What is the cause of this error