Function callable_quad 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn callable_quad(
    n: usize,
    xx: *const f64,
    q: *mut CompilerResult,
) -> f64

Interface for Sympy’s LowLevelCallable.

Safety

1. If the model cannot be compiled to a fast function, NULL is returned.
2. The resulting function lives as long as q does and should not be stored
    separately.