foo.c:1:1: error: unknown type name 'def'
    1 | def IsPrime(num):
      | ^~~
foo.c: In function 'IsPrime':
foo.c:1:17: error: expected declaration specifiers before ':' token
    1 | def IsPrime(num):
      |                 ^
foo.c:1:5: warning: type of 'num' defaults to 'int' [-Wimplicit-int]
    1 | def IsPrime(num):
      |     ^~~~~~~
foo.c:18: warning: control reaches end of non-void function [-Wreturn-type]

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