Task's precedence

[Mmorgese]

Before starting thanks for your work on pyschedule!

The problem is more complex then this, but it can be reduced to this simple scenario.
Running the code:
S = Scenario('S',horizon=1)
`
Ar=S.Resource('Ar')

At=S.Task('At', length=2, schedule_cost=-100, delay_cost=1)

Bt=S.Task('Bt', length=2, schedule_cost=-50, delay_cost=1)

At+=Ar

Bt+=Ar

S+= At<=Bt`

the solver choose to schedule Bt instead of At.
How can I make it schedule At instead of Bt?
The intention is use the schedule_cost as a priority in the choice of what tasks schedule if there is no enough time to make them all, but the case aforementioned brings to inconsistent schedule (in my scenario).