In file included from main.c:2:
main.c: In function ‘main’:
arena.h:136:20: error: implicit declaration of function ‘arena_strlen’; did you mean ‘arena_strdup’? [-Wimplicit-function-declaration]
  136 |         size_t n = arena_strlen(s);        \
      |                    ^~~~~~~~~~~~
main.c:15:5: note: in expansion of macro ‘arena_sb_append_cstr’
   15 |     arena_sb_append_cstr(&arena, &sb, "Hello, ");
      |     ^~~~~~~~~~~~~~~~~~~~