Subtract bug

[galharth]

this is the situation Im trying to stimulate:
https://cloud.githubusercontent.com/assets/236678/18207503/b61342e2-712b-11e6-881c-bb1970ab24bb.PNG
the data:

[
    [
        [
            [
                [0, 0],
                [0, 4],
                [4, 4],
                [4, 0],
                [0, 0]
            ]
        ]
    ],
    [
        [
            [
                [2, -2],
                [2, 5],
                [3, -2],
                [3, 5],
                [2, -2]
            ]
        ]
    ]
]

the code:

var subtract = require('gh-clipping-algorithm').subtract;
var json = require('./clippingtest.json');

console.log(JSON.stringify(subtract(json[0], json[1]),null,"  "));

output:
[[[[2.857142857142857,4],[2.142857142857143,4],[2,4],[0,4],[0,0],[2,0],[2.2857142857142856,0],[2.857142857142857,4]]],[[[3,4],[4,4],[4,0],[3,0],[3,4]]]]

The output is invalid..