preprocessor syntax - a few tips (IKM

Based on an IKM question and my research on P372 [[ARM]]

1) redefine and "undef" a name is fine. I feel the "undef" feature is rather useful but unavailable in the compiler.
2) can check if a macro is defined or not, using
#if defined AA
#ifdef AA
#if !defined AA
#ifndef AA

3) a macro can appear in another preprocessor code


#define AA2(jkl, xyz) {#xyz ## #jkl}
char result[] = AA2(def, abc); // will produce "abcdef" without space