bug#64212: 29.0.91; \Phi missing in TeX input method

[Roland Winkler]

On Thu, Jun 22 2023, Eli Zaretskii wrote:
> Why do you need that letter in a TeX context?  

The TeX input method is ideal if you are familar with TeX syntax, but
you have unicode-aware plain text (not a TeX source file) that may be
sprinkled with math symbols including greek characters.  For me, this
includes, for example, diary files and the text and annotation layers of
djvu documents.

> If all that's needs to be added to the input method is
>
>    ("\\Phi" ?Φ)
>
> then I can do that.

I am confused that latin-ltx.el treats variants of phi and epsilon
special such that the code provides a definition of \Epsilon (where I do
not see where this is done), but the definition of \Phi is missing.