function accumulator( x ) {
		var v;
		if ( arguments.length === 0 ) {
			v = mmean();
			if ( v === null ) {
				return v;
			}
			return 1.0 / v;
		}
		return 1.0 / mmean( 1.0/x );
	}
If provided a value, the accumulator function returns an updated harmonic mean. If not provided a value, the accumulator function returns the current harmonic mean.

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