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M4 rules it reads in the macro's arguments (if any) it determines the expansion of the macro and inserts this expansion at the beginning of its input m4 continues scanning the input, starting with the expansion

example: define(definenum',define(num',99')') num # -> num definenum num # -> define(num',99') num -> 99

Unless a nested macro is quoted, it is expanded immediately:

define(definenum', define(num', `99')) num # -> 99 definenum # ->