Seesaw balance

Decide which way a playground seesaw tips when children are standing on it.

The seesaw is a plank balanced on a pivot. A position is the signed distance from the pivot in decimetres: negative numbers are on the left half, positive on the right, and 0 is directly over the pivot. The plank itself is weightless and reaches half_length on each side.

Each child is [weight, position]. A child whose position is beyond the plank, abs(position) > half_length, is not on the seesaw and is ignored; a child standing exactly at half_length is on it.

Each child on the plank pushes their side down with a torque equal to weight * abs(position). A child over the pivot adds nothing. Add up the torque on each side. Then:

• if the left total is larger than the right total by more than tolerance, the answer is "left" (the left side goes down);
• if the right total is larger than the left total by more than tolerance, the answer is "right";
• otherwise (including an empty plank) the answer is "level".

Input: riders, a list of [weight, position] pairs; half_length, the plank's reach on each side; tolerance, the largest torque difference still considered level.

Output: one of the strings "left", "right" or "level".