docs: function(matrix) {
        return Matrix(this.a * matrix.a + this.c * matrix.b, this.b * matrix.a + this.d * matrix.b, this.a * matrix.c + this.c * matrix.d, this.b * matrix.c + this.d * matrix.d, this.a * matrix.tx + this.c * matrix.ty + this.tx, this.b * matrix.tx + this.d * matrix.ty + this.ty);
      }