A12.3 Macro Definition and Expansion
A control line of the form
#define identifier token-sequence
causes the preprocessor to replace subsequent instances of the identifier with the given
sequence of tokens; leading and trailing white space around the token sequence is dis-
carded. A second #define for the same identifier is erroneous unless the second token
sequence is identical to the first, where all white space separations are taken to be
equivalent
這段其中有一個小段，我看了許久才知道作者想要表達什麼，
A second #define for the same identifier is erroneous unless the second token
sequence is identical to the first
#define A 123
#define A 1234 <<<==== warning message