Hello Hans and developers,

I have been trying to convert some two- and three-step macros (as shown 
below) to single-step macros using the LMTX syntax given in the 
low-level macros manual. I have succeeded in many cases, but failed when 
the macro follows one particular group of patterns.

The group of patterns is similar to this macro to process

    \MyMacro[optional]{Required}{Required}{Optional}

In this the number of arguments can be 2, 3, or 4.

A three-step solution might be as follows, which I use for a list of 
persons:

    \defineregister[Bindex]
    \setupregister [Bindex][n=1,
                             balance=no,
                             method=default, % or New follows Newton!
                             compress=no, % yes if not note number+q
                             expansion=yes,
                             pagecommand=\gobbleoneargument
                            ]

    \starttexdefinition unexpanded Bi
       \dosingleempty\doBindex
    \stoptexdefinition

    \starttexdefinition unexpanded doBindex [#SORTAS]#INDEXED
       \def\SortAs{#SORTAS}%
       \def\Indexed{#INDEXED}%
       \dodoublegroupempty\doBindexFull
    \stoptexdefinition

    \starttexdefinition doBindexFull #BD#MORE
       \doifemptyelse{#MORE}
         {\Bindex[\SortAs]{\Indexed\ #BD}}
         {\Bindex[\SortAs]{\Indexed\ #BD\\ #MORE}}
    \stoptexdefinition

with input like:

    \Bi{Rumble, Walker}{1938|–|}%
    \Bi{Runia, David Theunis}{1951|–|}{Classicist}%
    \Bi{Rutherford, Ernest (Baron Rutherford of
    Nelson)}{1871|–|1937}{Physicist}%
    \Bi{Saenger, Paul}{1945|–|}%
    \Bi{Saldarini, Anthony J.}{1941|–|2001}%
    \Bi{Salter, William M.}{1853|–|1931}%
    \Bi[Saint-Exupery]{de Saint-Exupéry, Antoine}{1900|–|1944}%
    \Bi{Santayana, George}{1863|–|1952}%
    \Bi{Sapir, Edward}{1884|–|1939}{Linguist}%
    \Bi[Schrodinger]{Schrödinger, Erwin}{1887|–|1961}{Physicist}%
    \Bi[Spinoza]{de Spinoza, Baruch (Benedict de Spinoza, Bento de
    Espinosa)}{1632|–|1677}%

The best I can get with the newer syntax is a two-step:


    \starttexdefinition unexpanded Bi
       \dosingleempty\doBindex
    \stoptexdefinition
    \tolerant\def\doBindex [#1]#=#=#=%
       {\doifemptyelse{#4}
         {\Bindex[#1]{#2\ #3}}%
         {\Bindex[#1]{#2\ #3\\ #4}}%
       }

In the low-level macros manual, near the end of chapter 2, it says that 
some day there may be a use for #?, #!, #<, or #>. If I am not missing 
something in the manual about how to provide that first optional 
argument as an empty value, I think I have one. Could [#?] always return 
a value (perhaps empty)? That would allow something like:

    \tolerant\def\doBindex [#?]#=#=#=%
       {\doifemptyelse{#4}
         {\Bindex[#1]{#2\ #3}}%
         {\Bindex[#1]{#2\ #3\\ #4}}%
       }

-- 
Rik