function logpdf( x ) {
		var z;
		if ( isnan( x ) ) {
			return NaN;
		}
		if (
			x < mu - s ||
			x > mu + s
		) {
			return NINF;
		}
		z = ( x - mu ) / s;
		return ln( 1.0 + cospi( z ) ) - ln( 2.0 * s );
	}
Evaluates the logarithm of the probability density function (PDF) for a raised cosine distribution.

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

@example
var y = logpdf( -1.2 );
// returns <number>