m4 macro language http://mbreen.com/m4.html

comman line invoking

m4 -D<MACRO>

builtins

define(<macro>, <value>)
undefine(<macaro>)
ifdef(<macro>, <then>, <else>)
ifelse(<a>, <b>, <then>, <else>)
eval(<expr>)
len(<str>)
include(<filename>)
syscmd(<cmd>)
`'are quotes
# for comments

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                # ->