function accumulator( c, dist ) {
		var delta;
		var mu;
		var N;

		// Update number of data points belonging to a cluster:
		N = out.get( c, 0 ) + 1;
		out.set( c, 0, N );

		// Update the total sum of squared distances within a cluster:
		out.set( c, 1, out.get( c, 1 )+dist );

		// Update the cluster's squared distance mean and standard deviation (using Welford's algorithm):
		mu = out.get( c, 2 );
		delta = dist - mu;
		mu += delta / N;
		M2[ c ] += delta * ( dist-mu );
		out.set( c, 2, mu );
		out.set( c, 3, sqrt( M2[c]/(N-1) ) );

		return out;
	}
Updates cluster statistics.

@private
@param {NonNegativeInteger} c - cluster index to which a data point belongs
@param {number} dist - a data point's squared distance to its respective centroid
@returns {ndarray} matrix containing cluster statistics