code: function pdf( x, mu, b ) {
  	var lnb = ln( 2 * b );
  	var logd = - lnb - abs( x - mu ) / b;
  	return exp( logd );
  }
docs: PDF // 
  FUNCTION: pdf( x, mu, b )
  Evaluates the probability density function (PDF) for a Laplace distribution with location parameter `mu` and scale parameter `b` at a value `x`.
  
  @param {Number} x - input value
  @param {Number} mu - location parameter
  @param {Number} b - scale parameter
  @returns {Number} evaluated PDF