Candidate density acceptance-rejection-sampling

We want to generate a random sample from the posterior given in Exercise 3.1. First we draw a random sample of size 100000 from a Laplace 0,l) candidate density. Use acceptance-rejection-sampling to reshape it to be a random sample from the posterior. 

If the node is a single parameter and its conditional distribution is log-concave, we can draw an observation from the conditional distribution using the adaptive rejection sampling algorithm described in Chapter 2. Generally it takes only a few steps before we get an accepted draw from the conditional distribution, since we are tightening the candidate density with every unaccepted draw. 

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