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Message-ID:  <35e5f75d-508a-056f-45dd-c91c35083a29@gmail.com>
Date:         Mon, 24 Dec 2018 11:43:15 +0330
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              <LATEX-L@LISTSERV.UNI-HEIDELBERG.DE>
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From:         Tisha Shusho <tisha.shusho@GMAIL.COM>
Subject: What does this function place into the input stream?
To:           LATEX-L@LISTSERV.UNI-HEIDELBERG.DE
Precedence: list
Envelope-To: <rainer.schoepf@gmx.net>
Status: R

Hello List,

I am trying to understand the expl3 conventions by looking at its source
files. However, I am not sure what the final code that is placed
into the input stream by the following function (copied from
`expl3-code.tex', 2018-12-12) is:

\use:x
   {
     \prg_new_conditional:Npnn \exp_not:N \token_if_macro:N ##1
       { p , T ,  F , TF }
       {
         \exp_not:N \exp_after:wN \exp_not:N \__token_if_macro_p:w
         \exp_not:N \token_to_meaning:N ##1 \tl_to_str:n { ma : }
           \exp_not:N \q_stop
       }
     \cs_new:Npn \exp_not:N  \__token_if_macro_p:w
       ##1 \tl_to_str:n { ma } ##2 \c_colon_str ##3 \exp_not:N \q_stop
   }
       {
         \str_if_eq:nnTF { #2 } { cro }
           { \prg_return_true: }
           { \prg_return_false: }
       }

As I understand, after the first level of expansion it becomes:

\prg_new_conditional:Npnn \token_if_macro:N #1 % Line 1
   { p , T ,  F , TF }                          % Line 2
   {                                            % Line 3
     \exp_after:wN \__token_if_macro_p:w        % Line 4
     \token_to_meaning:N #1 ma:                 % Line 5
       \q_stop                                  % Line 6
   }                                            % Line 7
\cs_new:Npn \__token_if_macro_p:w              % Line 8
   #1 ma #2 : #3 \q_stop                        % Line 9
   {                                            % Line 10
     \str_if_eq:nnTF { #2 } { cro }             % Line 11
       { \prg_return_true: }                    % Line 12
       { \prg_return_false: }                   % Line 13
   }

The category codes of all characters in ma: (in line 5),
ma (in line 9), and : (in line 9) are 12.
Now let's suppose that the arguments are
\aaa whose meaning is "zzz", bbb, and x respectively.
So, after the second level of expansion, I think the
following piece of code is placed into the input stream:

\prg_new_conditional:Npnn \token_if_macro:N \aaa % Line 1
   { p , T ,  F , TF }                            % Line 2
   {                                              % Line 3
     \__token_if_macro_p:w                        % Line 4
     zzzma:                                       % Line 5
                                                  % Line 6
   }                                              % Line 7
\cs_new:Npn \__token_if_macro_p:w                % Line 8
   zzzmabbb:x                                     % Line 9
   {                                              % Line 10
     \str_if_eq:nnTF { bbb } { cro }              % Line 11
       { \prg_return_true: }                      % Line 12
       { \prg_return_false: }                     % Line 13
   }

Shouldn't the token "\aaa" in Line 1 and zzz in
line 5 and 9 be replaced with "#1", because the function
\prg_new_conditional:Npnn expects a parameter to be there?

Also, I am not quite sure about the expansion in Line 9.
Any help would be gratefully appreciated.

Thanks,
Tisha