foo.c: In function 'M':
foo.c:27:9: error: 'm' undeclared (first use in this function)
   27 |         m = x * y * temp;
      |         ^
foo.c:27:9: note: each undeclared identifier is reported only once for each function it appears in
foo.c:29:1: warning: control reaches end of non-void function [-Wreturn-type]
   29 | }
      | ^

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