How to write a poisson shock (varexo) mu?

I want to write an exogenous shock with a two-state poisson distribution,i.e mu={mu1.mu2}. For half the possibility(0.5), mu=mu1, and for the other half, mu=mu2. So could you please tell me how to write it on Dynare?

Thank you very much for your help!
Aurelia

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