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+% language=us
+
+% This feature was done mid May 2020 with Alien Chatter (I really need to find the
+% original cd's) in the background.
+
+\environment evenmore-style
+
+\startcomponent evenmore-parameters
+
+\startchapter[title=Parameters]
+
+When \TEX\ reads input it either does something directly, like setting a
+register, loading a font, turning a character into a glyph node, packaging a box,
+or it sort of collects tokens and stores them somehow, in a macro (definition),
+in a token register, or someplace temporary to inject them into the input later.
+Here we'll be discussing macros, which have a special token list containing the
+preamble defining the arguments and a body doing the real work. For instance when
+you say:
+
+\starttyping[option=TEX]
+\def\foo#1#2{#1 + #2 + #1 + #2}
+\stoptyping
+
+the macro \type {\foo} is stored in such a way that it knows how to pick up the
+two arguments and when expanding the body, it will inject the collected arguments
+each time a reference like \type {#1} or \type {#2} is seen. In fact, quite
+often, \TEX\ pushes a list of tokens (like an argument) in the input stream and
+then detours in taking tokens from that list. Because \TEX\ does all its memory
+management itself the price of all that copying is not that high, although during
+a long and more complex run the individual tokens that make the forward linked
+list of tokens get scattered in token memory and memory access is still the
+bottleneck in processing.
+
+A somewhat simplified view of how a macro like this gets stored is the following:
+
+\starttyping
+hash entry "foo" with property "macro call" =>
+
+    match (# property stored)
+    match (# property stored)
+    end of match
+
+    match reference 1
+    other character +
+    match reference 2
+    other character +
+    match reference 1
+    other character +
+    match reference 2
+\stoptyping
+
+When a macro gets expanded, the scanner first collects all the passed arguments
+and then pushes those (in this case two) token lists on the parameter stack. Keep
+in mind that due to nesting many kinds of stacks play a role. When the body gets
+expanded and a reference is seen, the argument that it refers to gets injected
+into the input, so imagine that we have this definition:
+
+\starttyping[option=TEX]
+\foo#1#2{\ifdim\dimen0=0pt #1\else #2\fi}
+\stoptyping
+
+and we say:
+
+\starttyping[option=TEX]
+\foo{yes}{no}
+\stoptyping
+
+then it's as if we had typed:
+
+\starttyping[option=TEX]
+\ifdim\dimen0=0pt yes\else no\fi
+\stoptyping
+
+So, you'd better not have something in the arguments that messes up the condition
+parser! From the perspective of an expansion machine it all makes sense. But it
+also means that when arguments are not used, they still get parsed and stored.
+Imagine using this one:
+
+\starttyping[option=TEX]
+\def\foo#1{\iffalse#1\oof#1\oof#1\oof#1\oof#1\fi}
+\stoptyping
+
+When \TEX\ sees that the condition is false it will enter a fast scanning mode
+where it only looks at condition related tokens, so even if \type {\oof} is not
+defined this will work ok:
+
+\starttyping[option=TEX]
+\foo{!}
+\stoptyping
+
+But when we say this:
+
+\starttyping[option=TEX]
+\foo{\else}
+\stoptyping
+
+It will bark! This is because each \type {#1} reference will be resolved, so we
+effectively have
+
+\starttyping[option=TEX]
+\def\foo#1{\iffalse\else\oof\else\oof\else\oof\else\oof\else\fi}
+\stoptyping
+
+which is not good. On the other hand, since expansion takes place in quick
+parsing mode, this will work:
+
+\starttyping[option=TEX]
+\def\oof{\else}
+\foo\oof
+\stoptyping
+
+which actually is:
+
+\starttyping[option=TEX]
+\def\foo#1{\iffalse\oof\oof\oof\oof\oof\oof\oof\oof\oof\fi}
+\stoptyping
+
+So, a reference to an argument effectively is just a replacement. As long as you
+keep that in mind, and realize that while \TEX\ is skipping \quote {if} branches
+nothing gets expanded, you're okay.
+
+Most users will associate the \type {#} character with macro arguments or
+preambles in low level alignments, but since most macro packages provide a higher
+level set of table macros the latter is less well known. But, as often with
+characters in \TEX, you can do magic things:
+
+\starttyping[option=TEX]
+\catcode`?=\catcode`#
+
+\def\foo #1#2?3{?1?2?3} \meaning\foo\space=>\foo{1}{2}{3}\par
+\def\foo ?1#2?3{?1?2?3} \meaning\foo\space=>\foo{1}{2}{3}\par
+\def\foo ?1?2#3{?1?2?3} \meaning\foo\space=>\foo{1}{2}{3}\par
+\stoptyping
+
+Here the question mark also indicates a macro argument. However, when expanded
+we see this as result:
+
+\starttyping
+macro:#1#2?3->?1?2?3 =>123
+macro:?1#2?3->?1?2?3 =>123
+macro:?1?2#3->#1#2#3 =>123
+\stoptyping
+
+The last used argument signal character (officially called a match character,
+here we have two that fit that category, \type {#} and \type {?}) is used in the
+serialization! Now, there is an interesting aspect here. When \TEX\ stores the
+preamble, as in our first example:
+
+\starttyping
+    match (# property stored)
+    match (# property stored)
+    end of match
+\stoptyping
+
+the property is stored, so in the later example we get:
+
+\starttyping
+    match (# property stored)
+    match (# property stored)
+    match (? property stored)
+    end of match
+\stoptyping
+
+But in the macro body the number is stored instead, because we need it as
+reference to the parameter, so when that bit gets serialized \TEX\ (or more
+accurately: \LUATEX, which is what we're using here) doesn't know what specific
+signal was used. When the preamble is serialized it does keep track of the last
+so|-|called match character. This is why we see this inconsistency in rendering.
+
+A simple solution would be to store the used signal for the match argument, which
+probably only takes a few lines of extra code (using a nine integer array instead
+of a single integer), and use that instead. I'm willing to see that as a bug in
+\LUATEX\ but when I ran into it I was playing with something else: adding the
+ability to prevent storing unused arguments. But the resulting confusion can make
+one wonder why we do not always serialize the match character as \type {#}.
+
+It was then that I noticed that the preamble stored the match tokens and not the
+number and that \TEX\ in fact assumes that no mixture is used. And, after
+prototyping that in itself trivial change I decided that in order to properly
+serialize this new feature it also made sense to always serialize the match token
+as \type {#}. I simply prefer consistency over confusion and so I caught two
+flies in one stroke. The new feature is indicated with a \type {#0} parameter:
+
+\startbuffer
+
+\bgroup
+\catcode`?=\catcode`#
+
+\def\foo ?1?0?3{?1?2?3} \meaning\foo\space=>\foo{1}{2}{3}\crlf
+\def\foo ?1#0?3{?1?2?3} \meaning\foo\space=>\foo{1}{2}{3}\crlf
+\def\foo #1#2?3{?1?2?3} \meaning\foo\space=>\foo{1}{2}{3}\crlf
+\def\foo ?1#2?3{?1?2?3} \meaning\foo\space=>\foo{1}{2}{3}\crlf
+\def\foo ?1?2#3{?1?2?3} \meaning\foo\space=>\foo{1}{2}{3}\crlf
+\egroup
+\stopbuffer
+
+\typebuffer[option=TEX]
+
+\start
+\getbuffer
+\stop
+
+So, what is the rationale behind this new \type{#0} variant? Quite often you
+don't want to do something with an argument at all. This happens when a macro
+acts upon for instance a first argument and then expands another macro that
+follows up but only deals with one of many arguments and discards the rest. Then
+it makes no sense to store unused arguments. Keep in mind that in order to use it
+more than once an argument does need to be stored, because the parser only looks
+forward. In principle there could be some optimization in case the tokens come
+from macros but we leave that for now. So, when we don't need an argument, we can
+avoid storing it and just skip over it. Consider the following:
+
+\startbuffer
+\def\foo     #1{\ifnum#1=1 \expandafter\fooone\else\expandafter\footwo\fi}
+\def\fooone#1#0{#1}
+\def\footwo#0#2{#2}
+\foo{1}{yes}{no}
+\foo{0}{yes}{no}
+\stopbuffer
+
+\typebuffer[option=TEX]
+
+We get:
+
+\getbuffer
+
+Just for the record, tracing of a macro shows that indeed there is no argument
+stored:
+
+\starttyping[option=TEX]
+\def\foo#1#0#3{....}
+\foo{11}{22}{33}
+\foo #1#0#3->....
+#1<-11
+#2<-
+#3<-33
+\stoptyping
+
+Now, you can argue, what is the benefit of not storing tokens? As mentioned
+above, the \TEX\ engines do their own memory management. \footnote {An added
+benefit is that dumping and undumping is relatively efficient too.} This has
+large benefits in performance especially when one keeps in mind that tokens get
+allocated and are recycled constantly (take only lookahead and push back).
+
+However, even if this means that storing a couple of unused arguments doesn't put
+much of a dent in performance, it does mean that a token sits somewhere in memory
+and that this bit of memory needs to get accessed. Again, this is no big deal on
+a computer where a \TEX\ job can take one core and basically is the only process
+fighting for \CPU\ cache usage. But less memory access might be more relevant in
+a scenario of multiple virtual machines running on the same hardware or multiple
+\TEX\ processes on one machine. I didn't carefully measure that so I might be
+wrong here. Anyway, it's always good to avoid moving around data when there is no
+need for it.
+
+Just to temper expectations with respect to performance, here are some examples:
+
+\starttyping[option=TEX]
+\catcode`!=9 % ignore this character
+\firstoftwoarguments
+  {!!!!!!!!!!!!!!!!!!!}{!!!!!!!!!!!!!!!!!!!}
+\secondoftwoarguments
+  {!!!!!!!!!!!!!!!!!!!}{!!!!!!!!!!!!!!!!!!!}
+\secondoffourarguments
+  {!!!!!!!!!!!!!!!!!!!}{!!!!!!!!!!!!!!!!!!!}
+  {!!!!!!!!!!!!!!!!!!!}{!!!!!!!!!!!!!!!!!!!}
+\stoptyping
+
+In \CONTEXT\ we define these macros as follows:
+
+\starttyping[option=TEX]
+\def\firstoftwoarguments      #1#2{#1}
+\def\secondoftwoarguments     #1#2{#2}
+\def\secondoffourarguments#1#2#3#4{#2}
+\stoptyping
+
+The performance of 2 million expansions is the following (probably half or less
+on a more modern machine):
+
+\starttabulate[||||]
+\BC macro                          \BC total \BC step        \NC \NR
+\NC \type {\firstoftwoarguments}   \NC 0.245 \NC 0.000000123 \NC \NR
+\NC \type {\secondoftwoarguments}  \NC 0.251 \NC 0.000000126 \NC \NR
+\NC \type {\secondoffourarguments} \NC 0.390 \NC 0.000000195 \NC \NR
+\stoptabulate
+
+But we could use this instead:
+
+\starttyping[option=TEX]
+\def\firstoftwoarguments      #1#0{#1}
+\def\secondoftwoarguments     #0#2{#2}
+\def\secondoffourarguments#0#2#0#0{#2}
+\stoptyping
+
+which gives:
+
+\starttabulate[||||]
+\BC macro                          \BC total \BC step        \NC \NR
+\NC \type {\firstoftwoarguments}   \NC 0.229 \NC 0.000000115 \NC \NR
+\NC \type {\secondoftwoarguments}  \NC 0.236 \NC 0.000000118 \NC \NR
+\NC \type {\secondoffourarguments} \NC 0.323 \NC 0.000000162 \NC \NR
+\stoptabulate
+
+So, no impressive differences, especially when one considers that when that many
+expansions happen in a run, getting the document itself rendered plus expanding
+real arguments (not something defined to be ignored) will take way more time
+compared to this. I always test an extension like this on the test suite
+\footnote {Currently some 1600 files that take 24 minutes plus or minus 30
+seconds to process on a high end 2013 laptop. The 260 page manual with lots of
+tables, verbatim and \METAPOST\ images takes around 11 seconds. A few
+milliseconds more or less don't really show here. I only time these runs because
+I want to make sure that there are no dramatic consequences.} as well as the
+\LUAMETATEX\ manual (which takes about 11 seconds) and although one can notice a
+little gain, it makes more sense not to play music on the same machine as we run
+the \TEX\ job, if gaining milliseconds is that important. But, as said, it's more
+about unnecessary memory access than about \CPU\ cycles.
+
+This extension is downward compatible and its overhead can be neglected. Okay,
+the serialization now always uses \type {#} but it was inconsistent before, so
+I'm willing to sacrifice that (and I'm pretty sure no \CONTEXT\ user cares or
+will even notice). Also, it's only in \LUAMETATEX\ (for now) so that other macro
+packages don't suffer from this patch. The few cases where \CONTEXT\ can benefit
+from it are easy to isolate for \MKIV\ and \LMTX\ so we can support \LUATEX\ and
+\LUAMETATEX.
+
+I mentioned \LUATEX\ and how it serializes, but for the record, let's see how
+\PDFTEX, which is very close to original \TEX\ in terms of source code, does it.
+If we have this input:
+
+\starttyping[option=TEX]
+\catcode`D=\catcode`#
+\catcode`O=\catcode`#
+\catcode`N=\catcode`#
+\catcode`-=\catcode`#
+\catcode`K=\catcode`#
+\catcode`N=\catcode`#
+\catcode`U=\catcode`#
+\catcode`T=\catcode`#
+\catcode`H=\catcode`#
+
+\def\dek D1O2N3-4K5N6U7T8H9{#1#2#3 #4#6#7#8#9}
+
+{\meaning\dek \tracingall \dek don{}knuth}
+\stoptyping
+
+The meaning gets typeset as:
+
+\starttyping
+macro:D1O2N3-4K5N6U7T8H9->H1H2H3 H4H6H7H8H9don nuth
+\stoptyping
+
+while the tracing reports:
+
+\starttyping
+\dek D1O2N3-4K5N6U7T8H9->H1H2H3 H5H6H7H8H9
+D1<-d
+O2<-o
+N3<-n
+-4<-
+K5<-k
+N6<-n
+U7<-u
+T8<-t
+H9<-h
+\stoptyping
+
+The reason for the difference, as mentioned, is that the tracing uses the
+template and therefore uses the stored match token, while the meaning uses the
+reference match tokens that carry the number and at that time has no access to
+the original match token. Keeping track of that for the sake of tracing would not
+make sense anyway. So, traditional \TEX, which is what \PDFTEX\ is very close to,
+uses the last used match token, the \type {H}. Maybe this example can convince
+you that dropping that bit of log related compatibility is not that much of a
+problem. I just tell myself that I turned an unwanted side effect into a new
+feature.
+
+\subject{A few side notes}
+
+The fact that characters can be given a special meaning is one of the charming
+properties of \TEX. Take these two cases:
+
+\starttyping[option=TEX]
+\bgroup\catcode`\&=5 &\egroup
+\bgroup\catcode`\!=5 !\egroup
+\stoptyping
+
+In both lines there is now an alignment character used outside an alignment. And,
+in both cases the error message is similar:
+
+\starttyping
+! Misplaced alignment tab character &
+! Misplaced alignment tab character !
+\stoptyping
+
+So, indeed the right character is shown in the message. But, as soon as you ask
+for help, there is a difference: in the first case the help is specific for a tab
+character, but in the second case a more generic explanation is given. Just try
+it.
+
+The reason is an explicit check for the ampersand being used as tab character.
+Such is the charm of \TEX. I'll probably opt for a trivial change to be
+consistent here, although in \CONTEXT\ the ampersand is just an ampersand so no
+user will notice.
+
+There are a few more places where, although in principle any character can serve
+any purpose, there are hard coded assumptions, like \type {$} being used for
+math, so a missing dollar is reported, even if math started with another
+character being used to enter math mode. This makes sense because there is no
+urgent need to keep track of what specific character was used for entering math
+mode. An even stronger argument could be that \TEX ies expect dollars to be used
+for that purpose. Of course this works fine:
+
+\starttyping[option=TEX]
+\catcode`€=\catcode`$
+€ \sqrt{x^3} €
+\stoptyping
+
+But when we forget an \type {€} we get messages like:
+
+\starttyping
+! Missing $ inserted
+\stoptyping
+
+or more generic:
+
+\starttyping
+! Extra }, or forgotten $
+\stoptyping
+
+which is definitely a confirmation of \quotation {America first}. Of course we
+can compromise in display math because this is quite okay:
+
+\starttyping[option=TEX]
+\catcode`€=\catcode`$
+$€ \sqrt{x^3} €$
+\stoptyping
+
+unless of course we forget the last dollar in which case we are told that
+
+\starttyping
+! Display math should end with $$
+\stoptyping
+
+so no matter what, the dollar wins. Given how ugly the Euro sign looks I can live
+with this, although I always wonder what character would have been taken if \TEX\
+was developed in another country.
+
+\stopchapter
+
+\stopcomponent