GPU Credits Simulation

Implement a credit system that tracks GPU credits over time. Three operations must be supported, all tagged with a monotonically increasing timestamp:

1. add(t, x, t_expire) – at time t, add x credits that will expire at time t_expire (t_expire > t).
2. cost(t, x) – at time t, attempt to consume x credits. If at least x credits are currently available (after all earlier operations), deduct them and return True; otherwise return False and change nothing.
3. expire – credits automatically become unusable as soon as the current time is ≥ their expiration time; no explicit call is required.

The system must process a stream of these calls in the order they are given and, for every cost operation, immediately return whether it succeeded. Internally you must deduct the oldest still-valid credits first (FIFO order among non-expired batches). You may assume that all operation timestamps are strictly increasing and that every t_expire is greater than the add time. Implement the class GpuCreditManager with methods add, cost, and any helper state you need.