Transform a tex source so that all macros are replaced by their definition

Question

Is it possible to see the output of the TeX ‘pre-processor’, i. e. the intermediate step before the actual output is done but with all user-defined macros replaced and only

Answer 1

There is no "pre-processor" in TeX. The replacement text for any control sequence at any stage can vary (this is used for a lot of things!). For example

\def\demo{\def\demo{cde}}
\demo

will first define \demo in one way and then change it. In the same way, you can redirect TeX primitives. For example, the LaTeX kernel moves \input to an internal position and alters it. A simplified version:

\let\@@input\input
\def\input#1{\@@input#1 }