module my_ifaces
  implicit none
contains
  subroutine expert(array, len, abstol, reltol)
    real :: array(:)
    integer :: len
    real, optional :: abstol, reltol
    real :: abstol0, reltol0

    abstol0 = 1e-6
    if ( present(abstol) ) abstol0 = abstol
    reltol0 = 1e-8
    if ( present(reltol) ) reltol0 = reltol

    ! code using abstol0 and reltol0
    print *, 'abstol =', abstol0
    print *, 'reltol =', reltol0
  end subroutine expert

  subroutine use_iface
    real :: array(100)
    call expert(array, 100)       
    ! uses abstol = 1e-6, reltol = 1e-8
    call expert(array, 100, 1e-3) 
    ! uses abstol = 1e-3, reltol = 1e-8
    call expert(array, 100, reltol=1e-3)
    ! uses abstol = 1e-6, reltol = 1e-3
  end subroutine use_iface
end module my_ifaces