al_ustr_insert - Allegro 5 API

       Insert  us2 into us1 beginning at byte offset pos.  pos cannot be less than 0.  If pos is past the end of
       us1 then the space between the end of the string and pos will be padded with NUL ('\0') bytes.

       If required, use al_ustr_offset(3alleg5) to find the byte offset for a given code point index.

       Returns true on success, false on error.

       al_ustr_insert - Allegro 5 API

al_ustr_insert_cstr(3alleg5),            al_ustr_insert_chr(3alleg5),            al_ustr_append(3alleg5),
       al_ustr_offset(3alleg5)

Allegro reference manual                                                                 al_ustr_insert(3alleg5)

              #include <allegro5/allegro.h>

              bool al_ustr_insert(ALLEGRO_USTR *us1, int pos, const ALLEGRO_USTR *us2)