Delimited \def's and expansion

Question

I try to understand how work the macro named \separator in MWE .

\documentclass[]{article}

\def\separator#1\separator#2{%
\unexpanded{#1}%
\ifx*#2%
\else
, \expandafter\separator\expandafter#2%
\fi
}%



    \def\stcs{\separator One\separator Two\separator three}

    \tracingmacros 1
    \begin{document}

    \stcs\separator*

    \end{document}

Trace of \separator is following:

\stcs ->\separator One\separator Two\separator three

\separator #1\separator #2->\unexpanded {#1}\ifx *#2\else , \expandafter \separator \expandafter #2\fi 
#1<-One
#2<-T

\separator #1\separator #2->\unexpanded {#1}\ifx *#2\else , \expandafter \separator \expandafter #2\fi 
#1<-Two
#2<-t

\separator #1\separator #2->\unexpanded {#1}\ifx *#2\else , \expandafter \separator \expandafter #2\fi 
#1<-three
#2<-*

What have I see.

At the first stage:

---

First: \separator gets a #1 as 'One' -this is normal for me. Second: \separator gets a #2 only 'T' but not 'Two'. Why?

At the second stage:

---

First: \separator gets a #1 as 'Two'. Why not 'T', as one can see from the first stage? And what had become of this 'T'? It lost?

\LaTeX pdf output is normal "One, Two, three". Thus, what is happen with #2?

Answer 1

#1 is a delimited argument that consists of all tokens up to the specified delimiter, \separator here. LaTeX doesn't really support the definition of such arguments (hence the use of \def rather than \newcommand.

#2 is a normal non-delimited argument. So this takes a single token or {} group. In the case you trace it just gets T just as if you go \fbox abc the argument of \fbox is just a.

#2 is just used for testing and if you are not at the end it is put back here:

\expandafter\separator\expandafter#2%

so when that \separator is expanded it sees

Two\separator three

in the input stream The T having been put back, so takes Two as #1 and t as #2 and the loop continues.