From PyMC3 to Edward, graph a->y<-b with all variables categoricals

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import pymc3 as pm3
import numpy as np
import theano
import theano.tensor as tt

arr_y = np.ones(shape=(3, 3, 3))*1/3
arr_ys = theano.shared(arr_y)

# define model
basic_model = pm3.Model()
with basic_model:
    a = pm3.Categorical('a', p=np.array([0.25, .30, .45]), observed = np.array([0, 0]))
    b = pm3.Categorical('b', p=np.array([0.6, .3, .1]))
    y = pm3.Categorical('y', p=tt.squeeze(arr_ys[a, b, :]))

# sample
with basic_model:      
    trace = pm3.sample(100)

# plot
pm3.traceplot(trace,figsize=(20,5));

The above model in PyMC3/Theano works fine. How would I translate it to Edward/Tensorflow