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Some number-like constructs which are not really valid as numeric constants are treated as numbers in preprocessing directives. If these constructs appear outside of preprocessing, they are erroneous. See Preprocessing Tokens.
Sometimes we need to insert spaces to separate tokens so that they
won’t be combined into a single number-like construct. For example,
0xE+12 is a preprocessing number that is not a valid numeric
constant, so it is a syntax error. If what we want is the three
tokens 0xE + 12, we have to insert two spaces as separators.