foo.c:1:1: error: unknown type name 'def'
    1 | def MergeSubs(arr):
      | ^~~
foo.c: In function 'MergeSubs':
foo.c:1:19: error: expected declaration specifiers before ':' token
    1 | def MergeSubs(arr):
      |                   ^
foo.c:1:5: warning: type of 'arr' defaults to 'int' [-Wimplicit-int]
    1 | def MergeSubs(arr):
      |     ^~~~~~~~~
foo.c:39: warning: control reaches end of non-void function [-Wreturn-type]

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