Defining macros within macros

The way to think of this is that ## gets replaced by # in just the same way that #1 gets replaced by ‘whatever is the first argument’.

So if you define a macro and use it as:

\def\a#1{+++#1+++#1+++#1+++}  \a{b}

the macro expansion produces ‘+++b+++b+++b+++’, which people find normal. However, if we now replace part of the macro:

\def\a#1{+++#1+++\def\x #1{xxx#1}}

\a{b} will expand to ‘+++b+++\def\x b{xxxb}’. This defines \x to be a macro delimited by b, and taking no arguments, which people may find strange, even though it is just a specialisation of the example above. If you want \a to define \x to be a macro with one argument, you need to write:

\def\a#1{+++#1+++\def\x ##1{xxx##1}}

and \a{b} will expand to ‘+++b+++\def\x #1{xxx#1}’, because #1 gets replaced by ‘b’ and ## gets replaced by #.

To nest a definition inside a definition inside a definition then you need ####1, as at each stage ## is replaced by #. At the next level you need 8 #s each time, and so on.

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