If A, B, C and D are the vertices of a square, find `vec(AB) + vec(BC)+ vec(CD) + vec(DA)`.

Explanation

Let ABCD be a square
Then `vec(AB)` = `vec(DC)` and `vec(AD)` = `vec(BC)`
Now `vec(AB) + vec(BC) + vec(CD) + vec(DA)`.
`vec(AB) + vec(BC) - vec(AB) - vec(BC)` = 0