is_lower Function

   elemental function is_lower(self)
   !< Return true if all characters in the string are lowercase.
   !<
   !<```fortran
   !< type(string) :: astring
   !< logical      :: test_passed(3)
   !< astring = ' Hello World'
   !< test_passed(1) = astring%is_lower().eqv..false.
   !< astring = ' HELLO WORLD'
   !< test_passed(2) = astring%is_lower().eqv..false.
   !< astring = ' hello world'
   !< test_passed(3) = astring%is_lower().eqv..true.
   !< print '(L1)', all(test_passed)
   !<```
   !=> T <<<
   class(string), intent(in) :: self     !< The string.
   logical                   :: is_lower !< Result of the test.
   integer                   :: c        !< Character counter.

   is_lower = .false.
   if (allocated(self%raw)) then
      is_lower = .true.
      do c=1, len(self%raw)
         if (index(UPPER_ALPHABET, self%raw(c:c))>0) then
            is_lower = .false.
            exit
         endif
      enddo
   endif
   endfunction is_lower