function accumulator( x ) {
		if ( arguments.length === 0 ) {
			return ( v === void 0 ) ? null : v;
		}
		v = 1.0 / mean( 1.0/x );
		return v;
	}
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] - new value
@returns {(number|null)} harmonic mean or null