function pdf( x, n ) {
	var mlim;
	if (
		isnan( x ) ||
		!isPositiveInteger( n ) ||
		!isfinite( n )
	) {
		return NaN;
	}
	if ( !isInteger( x ) ) {
		return 0.0;
	}
	mlim = ( n * ( n + 1 ) ) / 2;
	if ( x < 0.0 || x > mlim ) {
		return 0.0;
	}
	return exp( ln( weights( x, n ) ) - ( n * LN2 ) );
}
Evaluates the probability density function (PDF) of the Wilcoxon signed rank test statistic with `n` observations.

@param {number} x - input value
@param {PositiveInteger} n - number of observations
@returns {Probability} evaluated PDF

@example
var y = pdf( 7.0, 9 );
// returns ~0.01

@example
var y = pdf( 7.0, 6 );
// returns ~0.063

@example
var y = pdf( -1.0, 40 );
// returns 0.0

@example
var y = pdf( NaN, 10 );
// returns NaN

@example
var y = pdf( 0.0, NaN );
// returns NaN

@example
var y = pdf( 2.0, -1 );
// returns NaN

@example
var y = pdf( 2.0, 1.8 );
// returns NaN