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#ifdef FOO_LIBRARY # error Unexpected FOO_LIBRARY #endif #ifdef BAR_USE_BANG # ifndef BANG_LIBRARY # error Expected BANG_LIBRARY # endif # include "bang.h" #else # ifdef BANG_LIBRARY # error Unexpected BANG_LIBRARY # endif #endif #include "bar.h" int bar() { #ifdef BAR_USE_BANG return bang(); #else return 0; #endif }