function accumulator( x ) {
		if ( arguments.length === 0 ) {
			if ( N < 4 ) {
				return ( isnan( M4 ) ) ? NaN : null;
			}
			// Calculate the population excess kurtosis:
			g2 = (( N*M4 ) / ( M2*M2 )) - 3.0;

			// Return the corrected sample excess kurtosis:
			return (N-1) / ( (N-2)*(N-3) ) * ( ((N+1)*g2) + 6.0 );
		}
		N += 1;
		delta = x - mean;
		deltaN = delta / N;
		deltaN2 = deltaN * deltaN;

		term1 = delta * deltaN * (N-1);

		tmp = term1 * deltaN2 * ((N*N) - (3*N) + 3);
		tmp += 6.0 * deltaN2 * M2;
		tmp -= 4.0 * deltaN * M3;
		M4 += tmp;

		tmp = term1 * deltaN * (N-2);
		tmp -= 3.0 * deltaN * M2;
		M3 += tmp;

		M2 += term1;
		mean += deltaN;
		if ( N < 4 ) {
			return ( isnan( M4 ) ) ? NaN : null;
		}
		// Calculate the population excess kurtosis:
		g2 = (N*M4 / ( M2*M2 )) - 3.0;

		// Return the corrected sample excess kurtosis:
		return (N-1) / ( (N-2)*(N-3) ) * ( ((N+1)*g2) + 6.0 );
	}
If provided a value, the accumulator function returns an updated corrected sample excess kurtosis. If not provided a value, the accumulator function returns the current corrected sample excess kurtosis.

@private
@param {number} [x] - new value
@returns {(number|null)} corrected sample excess kurtosis