I am trying to understand a bit more what is going on when the Gaussian Process predict() method is called using a trace as the point. The trace object has samples from several random variables in it, but I only get a single mean and variance back. How do we designate which variable this is predicting, or why in this case do I not receive a vector back, for each value in X_predict?

Eg:

```
trace = pm.sample(draws=1000)
mu, var = gp.predict(x_pred, point=trace, diag=True)
```

Result: `mu = [0.83], var= [0.03]`

trace: `<MultiTrace: 4 chains, 1000 iterations, 5 variables>`

x_pred: `<ndarray, shape (1,50)>`

Thanks!