AWilco

// [B](f: (A) ⇒ [B]): [B]  ; Although the types in the arrays aren't strict (:
Object.defineProperties(Array.prototype, {
    'flatMap': {
        value: function (lambda) {
            return Array.prototype.concat.apply([], this.map(lambda));
        },
        writeable: false,
        enumerable: false
    }
});