def add_heat(heatmap, bbox_list):
    """
    Given a heatmap, add one to each pixel inside each bounding box in the
    specified list.

    Args:
        heatmap: Modify and return this heatmap.
        bbox_list: List of bounding boxes for potential car locations.

    Returns:
        A modified heatmap
    """

    # Iterate through list of bboxes
    for box in bbox_list:
        # Add += 1 for all pixels inside each bbox
        # Assuming each "box" takes the form ((x1, y1), (x2, y2))
        heatmap[box[0][1]:box[1][1], box[0][0]:box[1][0]] += 1

    # Return updated heatmap
    return heatmap