Some commands can't be decomposed further in (La)TeX -- they are TeX primitives, basically the
elementary particles of TeX, from which other macros can be constructed to simplify the usage of (La)TeX.
In the end it is a compiler or interpreter that must be instructed to operate if has read
\openin. The situation is similar to, say, a
C - source file: A
C compiler must know what the basic commands
return mean -- they are keywords and can't be reduced further.
texdef \ifx will 'fail' in the sense that it can just report that
\ifx is 'defined' as
Looking up such constructs needs some experience and the knowledge that
texdoc tex will report much of Knuth's TeX
texdoc etex will give more insight of the 'new' e-TeX inventions introduced in the late 1990s
texdoc source2e provides information about the LaTeX core.
texdoc texbytopic will give a concise introduction how D.E. Knuth's version TeX works
texdoc pdftex is nice to read about the PDF extensions.
In a similar manner,
source3 are available for insights about
XeTeX and the
\ifdefined is a new primitive introduced by
e-TeX, it has no further definition other than what is written in the binary.
Please note that
pdfTeX introduces some more primitives such as
Unfortunately, compiling the
The TeXBook source is not possible still nowadays.