foo.c: In function 'main':
foo.c:18:9: warning: 'gets' is deprecated [-Wdeprecated-declarations]
   18 |         gets(s1);
      |         ^~~~
In file included from foo.c:1:
/nix/store/linb70pgp53f0kygxm5wk163dx22d681-glibc-2.37-45-dev/include/stdio.h:605:14: note: declared here
  605 | extern char *gets (char *__s) __wur __attribute_deprecated__;
      |              ^~~~
foo.c:19:9: warning: 'gets' is deprecated [-Wdeprecated-declarations]
   19 |         gets(s2);
      |         ^~~~
/nix/store/linb70pgp53f0kygxm5wk163dx22d681-glibc-2.37-45-dev/include/stdio.h:605:14: note: declared here
  605 | extern char *gets (char *__s) __wur __attribute_deprecated__;
      |              ^~~~
foo.c:20:9: error: 's1_' undeclared (first use in this function); did you mean 's1'?
   20 |         s1_=diff(s1);
      |         ^~~
      |         s1
foo.c:20:9: note: each undeclared identifier is reported only once for each function it appears in
foo.c:21:9: error: 's2_' undeclared (first use in this function); did you mean 's2'?
   21 |         s2_=diff(s2);
      |         ^~~
      |         s2

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