foo.c: In function 'sum':
foo.c:3:5: warning: type of 'a' defaults to 'int' [-Wimplicit-int]
 int sum(a, b, c)
     ^~~
foo.c:3:5: warning: type of 'b' defaults to 'int' [-Wimplicit-int]
foo.c:3:5: warning: type of 'c' defaults to 'int' [-Wimplicit-int]
foo.c: In function 'main':
foo.c:14:6: warning: unused variable 'choose' [-Wunused-variable]
  int choose = 1;
      ^~~~~~

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