Problem description

In the following snippet

the function argument i is unsigned, but i < 0 is checked. This currently generates a warning (at least with gcc 14). I think most often, indices in Cantera are defined as size_t, so the check should be superfluous. But if this is called in other language interfaces, an error message with a signed index might be more helpful to the user (if e.g. -1 is passed). What do you think?