I’m reading [Probabilistic Programming for Hackers] (https://nbviewer.jupyter.org/github/CamDavidsonPilon/Probabilistic-Programming-and-Bayesian-Methods-for-Hackers/blob/master/Chapter2_MorePyMC/Ch2_MorePyMC_PyMC3.ipynb) and i’m wondering why did he used the first few data points from trace as a posterior distribution?

```
step = pm.Metropolis()
trace = pm.sample(20000, step=step)
burned_trace=trace[1000:]
```