fn f(xxxxxxxxxxxxxxxxxx: fn(a, b, b) -> a,
     xxxxxxxxxxxxxxxxxx: fn() -> a,
     xxxxxxxxxxxxxxxxxx: fn(a, b, b),
     xxxxxxxxxxxxxxxxxx: fn(),
     xxxxxxxxxxxxxxxxxx: fn(a, b, b) -> !,
     xxxxxxxxxxxxxxxxxx: fn() -> !)
    where F1: Fn(a, b, b) -> a,
          F2: Fn(a, b, b),
          F3: Fn(),
          F4: Fn() -> u32
{
}