function accumulator( x, y ) {
		var r;
		if ( arguments.length === 0 ) {
			r = pcorr();
			if ( r === null ) {
				return r;
			}
			return 1.0 - r;
		}
		return 1.0 - pcorr( x, y );
	}
If provided a value, the accumulator function returns an updated sample correlation distance. If not provided a value, the accumulator function returns the current sample correlation distance.

@private
@param {number} [x] - input value
@param {number} [y] - input value
@returns {(number|null)} sample correlation distance or null