function incrnanmean() {
	var mean = incrmean();
	return accumulator;

	/**
	* 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
	*/
	function accumulator( x ) {
		if ( arguments.length === 0 || isnan( x ) ) {
			return mean();
		}
		return mean( x );
	}
}
Returns an accumulator function which incrementally computes an arithmetic mean, ignoring `NaN` values.

@returns {Function} accumulator function

@example
var accumulator = incrnanmean();

var mu = accumulator();
// returns null

mu = accumulator( 2.0 );
// returns 2.0

mu = accumulator( NaN );
// returns 2.0

mu = accumulator( -5.0 );
// returns -1.5

mu = accumulator();
// returns -1.5