function accumulator( x ) {
		if ( arguments.length === 0 ) {
			return ( m === void 0 ) ? null : m;
		}
		if ( m === void 0 ) {
			m = x;
		} else {
			m += alpha * ( x-m );
		}
		return m;
	}
If provided a value, the accumulator function returns an updated mean. If not provided a value, the accumulator function returns the current mean.

@private
@param {number} [x] - new value
@returns {(number|null)} mean value or null