How to add constraints to parameters in pymc3 model?

I followed the first tutorial in pymc3 concerning a linear regression. I tried to adapt it to a different function that is :

Y_obs = log (x+a)^2 / (x-c)^2

As you can see it could be preferable that c was negative. Thus I wonder how to define the prior for c with negative values. For example, an HalfNormal distribution but with only the negative values instead of the positive ones.

More generally I wonder if it is possible to set bounds to parameters ?

For example, using Stan (or pyStan) in the model set up you can write :

parameters {
    real<lower=0, upper=1> p;
}

to set p \in [0, 1].