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The sample implementation for parse-gecos replaces @'s with the fullname, but it's supposed to be ampersand (&).The spec makes it clear for non-errno-based error, a foreign-error with :code & :foreign-interface set to #f, and :error-set is 'error.   All other details of the exception are left to the implementation.   In the case of user-info and group-info, the underlying posix calls return NULL without an error, therefore one of these is raised.  I feel like the case of "not found" needs more specific treatment.   It's a case common enough in programming (ls prints the uid/gid numeric value if there is no associated user/group, for example), but this specification provides no means to portably distinguish this case from other possible non-errno errors.   What I am suggesting is the addition of a user/group-not-found-error? predicate to the SRFI.