<think>
Okay, I need to implement the path_toll function based on the step_cost function. Let me think about the problem.

The task says that every third step is free. So, for each step in the path, if its position (starting from 1) is a multiple of 3, it's free. Otherwise, it's charged at the step's cost.

First, the step_cost function takes a kind (like 'a', 'b', etc.) and returns the cost. But the problem doesn't specify what the cost is for each kind. Wait, the problem says that step_cost is supposed to return the cost for a step of a certain kind. But the problem doesn't give any specific values. Oh, but the problem says that the step_cost is part of the interface, so maybe the actual implementation of step_cost is not provided here. Wait, the user is asking me to implement the path_toll function, but the step_cost is part of the interface. Wait, the user's task is to implement path_toll, which uses step_cost. But the step_cost is a function that returns the cost for a step of a certain kind. However, the problem doesn't specify what the cost is for each kind. Wait, maybe the step_cost is a helper function that the user is supposed to implement, but in the problem statement, the user is only asking for path_toll. Wait, the user's task is to implement path_toll, which uses step_cost. But the step_cost is part of the interface. So, the user is providing the interface, and I need to implement the functions according to that.

Wait, the problem says that the step_cost is a function that returns the cost for a step of a certain kind. But the problem doesn't specify what the cost is. However, the problem says that the path_toll function should total the path, with every third step free. So, the step_cost function must return the cost for each step, and then path_toll sums those costs, but skips every third step.

Wait, but the problem says that the step_cost is part of the interface. So, the user is supposed to implement the step_cost function, but in the problem, the user is only asking for path_toll. Wait, no. The user's task is to implement the path_toll function, which uses step_cost. But the step_cost is part of the interface. So, the user is providing the interface, and I need to implement the functions according to that. But the step_cost is not given any specific implementation. Wait, the user's task is to implement path_toll, which uses step_cost. But the step_cost is a function that returns the cost for a step of a certain kind. However, the problem doesn't specify what the cost is for each kind. So, perhaps the step_cost is a placeholder, and the actual implementation is up to the user. But the problem says that the step_cost is part of the interface, so the user is supposed to implement it. But the user is only asking for path_toll. Wait, the user's task is to implement path_toll, which uses step_cost. So, the step_cost is a function that the user is supposed to implement, but the problem doesn't give any details. Wait, maybe the step_cost is a function that returns the cost for each step, and the path_toll function is supposed to sum those costs, but every third step is free.

Wait, the problem says that the path_toll function totals a path, except that every third step along it is free, counting the steps from one. So, for each step in the steps list, if the step's index (starting at 1) is divisible