+ # Most tools' exit codes cannot be trusted, so By default we ignore
+ # their exit code and check the merged file's modification time in
+ # check_unchanged() to determine whether or not the merge was
+ # successful. The return value from run_merge_cmd, by default, is
+ # determined by check_unchanged().
+ #
+ # When a tool's exit code can be trusted then the return value from
+ # run_merge_cmd is simply the tool's exit code, and check_unchanged()
+ # is not called.
+ #
+ # The return value of exit_code_trustable() tells us whether or not we
+ # can trust the tool's exit code.
+ #
+ # User-defined and built-in tools default to false.
+ # Built-in tools advertise that their exit code is trustable by
+ # redefining exit_code_trustable() to true.
+
+ exit_code_trustable () {
+ false
+ }
+
+