foo.c: In function 'main':
foo.c:7:4: error: 'm' undeclared (first use in this function)
    7 |    m=a;
      |    ^
foo.c:7:4: note: each undeclared identifier is reported only once for each function it appears in
foo.c:8:4: error: 'n' undeclared (first use in this function)
    8 |    n=b;
      |    ^

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