code: public static function compose(callable $g, callable $f) : callable
      {
          return function ($input) use ($g, $f) {
              return Functions::call($g, Functions::call($f, $input));
          };
      }
docs: Returns the composition of two functions.
  
  For f: A->B and g: B->C, composition is defined as the function h such that h(a) == g(f(a)) for each a.
  
  @param callable $g
  @param callable $f
  @return callable