_dot( g, x, y ) {

		return g[ 0 ] * x + g[ 1 ] * y;

	}
A 4D simplex noise method.

@param {number} x - The x coordinate.
@param {number} y - The y coordinate.
@param {number} z - The z coordinate.
@param {number} w - The w coordinate.
@return {number} The noise value.