Here is a MWE. I would want `x[2]`

to always be positive. Thanks again.

```
using ZigZagBoomerang, Distributions, ForwardDiff, LinearAlgebra, SparseArrays
data = rand(Normal(3,10),20)
g(x) =sum(logpdf.(Normal(x[1],x[2]),data))
function ∇ϕ(x, i)
d = -ForwardDiff.gradient(g,x)
d[i]
end
n= 2
t0 = 0.0
x0 = abs.(randn(n))
θ0 = rand([-1.0,1.0], n)
c = 1.0*ones(length(x0))
Z = ZigZag(sparse(I(n)), x0*0);
T = 200.0
trace, (tT, xT, θT), (acc, num) = pdmp(∇ϕ, t0, x0, θ0, T, c, Z; adapt=true)
```

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