The Tollgates of Marrowmere

The road out of Marrowmere runs through a line of gatehouses. Some gates take a toll from a passing cart; others hand out a bounty from the town chest.

You are given three arguments:

• purse — an integer, the coins the carter sets out with (0 <= purse <= cap).
• cap — an integer, the most coins the purse can hold.
• tolls — a list of integers, one per gatehouse, in the order they are met. A positive value is a toll charged; a value of zero or less is a bounty of that many coins paid to the carter (a value of 0 pays nothing).

The cart meets the gatehouses in order:

• At a gate with a positive value v: if the purse holds at least v coins, v coins are paid out of the purse and the gate is passed. Otherwise the cart is turned back at once — that gate is not passed, and no later gate is reached.
• At a gate with a value of zero or less: the bounty is added to the purse, and the gate is passed. If this would take the purse above cap, the purse is left holding exactly cap coins and the surplus is lost.

Write tollRun(purse, cap, tolls), which returns a list of exactly two integers: the number of gates passed, and the coins left in the purse when the journey ends.