module man_or_boy

implicit none

contains

  recursive integer function A(k,x1,x2,x3,x4,x5) result(res)
    integer, intent(in) :: k
    interface
      recursive integer function x1()
      end function
      recursive integer function x2()
      end function
      recursive integer function x3()
      end function
      recursive integer function x4()
      end function
      recursive integer function x5()
      end function
    end interface
    integer :: m
    if ( k <= 0 ) then
      res = x4()+x5()
    else
      m = k
      res = B()
    end if

  contains

    recursive integer function B() result(res)
      m = m-1
      res = A(m,B,x1,x2,x3,x4)
    end function B

  end function A


  recursive integer function one() result(res)
    res = 1
  end function

  recursive integer function minus_one() result(res)
    res = -1
  end function

  recursive integer function zero() result(res)
    res = 0
  end function

end module man_or_boy

program test
  use man_or_boy
  write (*,*) A(10,one,minus_one,minus_one,one,zero)
end program test