Hi,

I’m struggeling to create an expression containing Bessel functions. I need something like this: `f = Expression("std::cyl_bessel_i(1,x[0])", degree=2)`

but I get an compilation error that the Bessel function is not found.

I also tried to use the Boost lib version of the Bessel function with `f = Expression("boost::math::cyl_bessel_i(1,x[0])", degree=2)`

but this function is also not found.

MWE:

```
from dolfin import *
f = Expression("std::cyl_bessel_i(1,x[0])", degree=2)
print(f(1.23))
```

Does someone has a tip for me? Thanks!