Corner values

You will write three small functions for the corner point method. The checker tests them on regions with ties and with negative numbers.

The idea. For $Z = ax + by$, work out $Z$ at each corner. The best value is the largest for a maximum and the smallest for a minimum. Several corners may share the best value.

Your tasks.

1. Finish z_value(a, b, point). Return $ax + by$ at the point (x, y).
2. Finish corner_values(a, b, corners). Return the list of $Z$ values, one for each corner, in the same order.
3. Finish best(a, b, corners, sense). Return the best value and the list of corners that reach it. The text sense is "max" or "min".