module integral
  use common, only : dp
  implicit none

  public

  interface
     real(dp) pure function ivp_func(t, y)
       import :: dp
       real(dp), intent(in) :: t, y
     end function ivp_func
     ! module procedure :: rk4
  end interface
contains
  real(dp) function rk4 (x) result(y)
    real(dp), intent(in) :: x
    y = x
  end function rk4

  ! pure function rk4()
  ! end function rk4
end module integral