Function solve_separable 

pub fn solve_separable(
    g_x: &Expr,
    h_y: &Expr,
    xvar: &str,
    yvar: &str,
) -> Result<OdeSolution, OdeError>

Solve the separable ODE y' = g(x)·h(y) as ∫ dy/h(y) = ∫ g(x) dx + C. The two antiderivatives are computed by the CAS integrator; either being non-integrable yields OdeError::NotIntegrable.