What does the symbol f (as opposed to f1 or f2) in
on the page Dynamical, Hamiltonian, and 2nd Order ODE Solvers · DifferentialEquations.jl refer to? That is, how is the second order ODE defined?
I saw the example at 2nd Order ODEs in Julia using DifferentialEquations - Stack Overflow
Possibly, however, it might be valuable to make the documentation more explicit.