foo.c:1:1: error: unknown type name 'def'
    1 | def substitute_cipher(key, s):
      | ^~~
foo.c: In function 'substitute_cipher':
foo.c:1:30: error: expected declaration specifiers before ':' token
    1 | def substitute_cipher(key, s):
      |                              ^
foo.c:4:5: error: expected declaration specifiers before 'for'
    4 |     for char in s:
      |     ^~~
foo.c:1:5: warning: type of 'key' defaults to 'int' [-Wimplicit-int]
    1 | def substitute_cipher(key, s):
      |     ^~~~~~~~~~~~~~~~~
foo.c:1:5: warning: type of 's' defaults to 'int' [-Wimplicit-int]
foo.c:14: warning: control reaches end of non-void function [-Wreturn-type]

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