-----Description----- 
This task requires writing a Lean 4 method that calculates how much rainwater would be trapped by a terrain represented as an array of heights. Imagine rainwater falls onto a terrain with varying elevation levels. The water can only be trapped between higher elevation points.

Given an array of non-negative integers representing the elevation map where the width of each bar is 1 unit, calculate how much water can be trapped after it rains.

-----Input-----
The input consists of one array:
heights: An array of non-negative integers representing elevation levels.

-----Output-----
The output is an integer:
Returns the total amount of rainwater that can be trapped.

