Foton does not appear to check design strings for syntactic correctness
Foton does not appear to check that a design string is syntactically correct. It only appears to check if a filter created from the design string matches a filter created from second order sections. If the design string is not syntactically correct, Foton appears to create an identity filter for it. Hence, if a filter file has a design string that is not syntactically correct and no corresponding second order sections defined for it, then it loads the file without producing any warning or error message for that filter, because the identity filters created for each match.