function laplace( rand, mu, b ) {
	var u = rand() - 0.5;
	return mu - ( b*sign( u )*ln( 1.0 - (2.0*abs(u)) ) );
}
Returns a pseudorandom number drawn from a Laplace (double exponential) distribution.

@private
@param {PRNG} rand - PRNG for uniformly distributed numbers
@param {number} mu - mean
@param {PositiveNumber} b - scale parameter
@returns {number} pseudorandom number