sindex_string_string Function

   elemental function sindex_string_string(self, substring, back) result(i)
   !< Return the position of the start of the first occurrence of string `substring` as a substring in `string`, counting from one.
   !< If `substring` is not present in `string`, zero is returned. If the back argument is present and true, the return value is
   !< the start of the last occurrence rather than the first.
   !<
   !<```fortran
   !< type(string) :: string1
   !< type(string) :: string2
   !< logical      :: test_passed(2)
   !< string1 = 'Hello World Hello!'
   !< string2 = 'llo'
   !< test_passed(1) = string1%index(substring=string2)==index(string='Hello World Hello!', substring='llo')
   !< test_passed(2) = string1%index(substring=string2, back=.true.)==index(string='Hello World Hello!', substring='llo', &
   !<                                                                       back=.true.)
   !< print '(L1)', all(test_passed)
   !<```
   !=> T <<<
   class(string), intent(in)           :: self      !< The string.
   type(string),  intent(in)           :: substring !< Searched substring.
   logical,       intent(in), optional :: back      !< Start of the last occurrence rather than the first.
   integer                             :: i         !< Result of the search.

   if (allocated(self%raw)) then
      i = index(string=self%raw, substring=substring%raw, back=back)
   else
      i = 0
   endif
   endfunction sindex_string_string