/in/foo.c: In function 'solve':
/in/foo.c:19:9: error: 'true' undeclared (first use in this function)
  return true;
         ^~~~
/in/foo.c:19:9: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:20:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^