-----Description-----
This task requires writing a Lean 4 function that calculates how much water can be trapped between elevations after it rains. The input is a list of non-negative integers representing an elevation map. Each index traps water depending on the min of max heights to its left and right.

-----Input-----
- height: A list of natural numbers representing elevations.

-----Output-----
- A natural number: total units of water that can be trapped.

