function accumulator( x ) {
		if ( arguments.length === 0 ) {
			return summary;
		}
		summary.window = W;
		summary.sum = msum( x );
		summary.mean = mmean( x );
		summary.variance = mvariance( x );
		summary.stdev = mstdev( x );
		summary.min = mmin( x );
		summary.max = mmax( x );
		summary.range = mrange( x );
		summary.midrange = mmidrange( x );
		return summary;
	}
If provided a value, the accumulator function returns an updated summary. If not provided a value, the accumulator function returns the current summary.

@private
@param {number} [x] - new value
@returns {Object} summary