Explicit Interfaces for Internal Procedures

In F90, the above program may be

program impint
   implicit none
   integer, parameter :: n=4
   integer, dimension(n) :: iv=(/3,1,4,2/)
   integer :: iw, how
   how = -1
   call sort(iv, n, how)
   print *, iv(1:n), how
contains      
   subroutine sort(iv,n,how)
   integer, dimension(:), intent(in) :: iv(:)
   integer, intent(in) :: n
   integer, intent(inout) :: how
   ...................
   end subroutine sort   
end program impint

In this case the interface of sort with the main program is explicit.