try returning expr instead of opt extr? what does env do?

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https://github.com/Juice-jl/Dice.jl/blob/a0022d390a20830876e17678b88b277bf65bbc8d/examples/qc/demo_stlc.jl#L36

    ])
end

# TODO: try returning expr instead of opt extr? what does env do?
function gen_zero(env::Ctx, tau::DistI{Typ})
    match(tau, [
        "TBool" => ()       -> DistSome(DistBoolean(flip(0.5))),