function pdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( !isInteger( x ) ) {
			return 0.0;
		}
		if ( x < 0.0 || x > mlim ) {
			return 0.0;
		}
		return exp( ln( weights( x, n ) ) - ( n * LN2 ) );
	}
Evaluates the probability density function (PDF) for the distribution of the Wilcoxon signed rank test statistic.

@private
@param {number} x - input value
@returns {Probability} evaluated PDF

@example
var y = pdf( 2 );
// returns <number>