hi

```
with pm.Model(coords=coords) as varying_intercept:
floor_idx = pm.MutableData("floor_idx", floor_measure, dims="obs_id")
county_idx = pm.MutableData("county_idx", county, dims="obs_id")
# Priors
mu_a = pm.Normal("mu_a", mu=0.0, sigma=10.0)
sigma_a = pm.Exponential("sigma_a", 1)
# Random intercepts
alpha = pm.Normal("alpha", mu=mu_a, sigma=sigma_a, dims="county")
# Common slope
beta = pm.Normal("beta", mu=0.0, sigma=10.0)
# Model error
sd_y = pm.Exponential("sd_y", 1)
# Expected value
y_hat = alpha[county_idx] + beta * floor_idx
```

my question is: what the alpha[county_idx] means? any document with the syntax?alpha shape is shape(30,1),and the county_idx shape is (1000,1)