function accumulator( x, y ) {
		if ( arguments.length === 0 ) {
			if ( N === 0 ) {
				return null;
			}
			return abs( acc() );
		}
		N += 1;
		return abs( acc( x, y ) );
	}
If provided input values, the accumulator function returns an updated sample correlation coefficient. If not provided input values, the accumulator function returns the current sample correlation coefficient.

@private
@param {number} [x] - new value
@param {number} [y] - new value
@returns {(number|null)} sample absolute correlation coefficient or null