A Brief Introduction to Regular GELLMU

5.1.  The Markup Commands

There are several kinds of commands:

• A maximal string introduced by \ followed by a letter⁶ and otherwise consisting only of letters and numbers. The first number, if any, must not be “0” since such names are reserved for use by the syntactic translator.
  Command names are case sensitive. Such a command is a container, corresponding to an SGML element, if it is immediately followed, without intervening white space, by the character {. In that case the delimited zone of containment normally ends with the subsequent “balancing character” }. (LaTeX-like multiple argument/option chains deserve more discussion; for now it will suffice to point out that the use of the \anch command in this document for making “anchors” is an example, and, of course, LaTeX's \frac command is another example. For the present discussion these commands are considered to be containers.) Such a command corresponds to an SGML defined-empty element if it is followed immediately without intervening white space by the character ;. In any other case there is syntactical ambiguity. However, the syntactic translator will produce a corresponding SGML open tag unless the logical variable gellmu-xml-strict has been set. (This variable is normally not set for advanced GELLMU.)

• Certain single characters.
  \, {, }, ^, _, $, %, and ~ have command meanings that are similar to their meanings in LaTeX. The characters ; and : are ordinary characters that have special meaning at the end of a command name. The character # is also a special character used, as with LaTeX, in newcommand templates. In source for the didactic article document type any non-alpha-numeric ASCII character may be escaped (referenced for itself) with a named command, e.g. % may be referenced for itself as \pct;. This is necessary in order to provide delayed evaluation for ultimate translation to one of many possible ultimate formats.

  The following language meanings apply to both basic and regular markup:

  1. "\": Command introducer. Escape in basic mode: "\\". This escape is incorrect in regular mode. For the didactic article document type the escape is "\bsl;". For other document types one may resort to an entity reference if there is an aversion to the provision of a corresponding defined-empty element or if one lacks control of the document type.

  2. "{": Command argument opener. Escape: "\{".

  3. "}": Command argument closer. Escape: "\}".

  4. "[": Command option opener.

  5. "]": Command option closer.

  6. ";": Command terminator for commands corresponding to defined-empty SGML elements; otherwise an ordinary character. Its use as a command terminator is invisible and may be omitted optionally in some contexts. This syntax is analogous to the use of ‘;’ as an entity reference terminator in HTML.

  7. ":": Special purpose command terminator used for indicating the close of a command-zone; otherwise an ordinary character. Its use as a command terminator is invisible.

  8. "%": Comment introducer (in force to end of line).

  Any command is terminated by a non-alpha-numeric character. There is syntactic ambiguity unless the terminator is one of ‘{’, ‘[’, ‘;’, or ‘:’. This kind of syntactic ambiguity is not permitted in the direct editing for an XML document type with GELLMU input. The terminator can be a blank space, but, if so, the blank space becomes invisible.

  The following language meanings apply to regular markup with allusion to the didactic article document type.

  1. "~": Non-breaking interword space. Equivalent: \nbs;, cf.   in HTML.

  2. "^": Superscript command. Equivalent: \sup.

  3. "_": Subscript command. Equivalent: \sub.

  4. "&": Dual use: tabular cells and entity introducer. ‘&’ introduces an entity reference if it is followed by anything other than white space. It is used, as in LaTeX, as a tabular cell delineator when it is followed by white space.

  5. "$": Toggle inline math mode.
     Equivalent:

     "\tmath{ . . . }"

     Nearly equivalent:

     "\( . . . \)" or

     "\math{ . . . }".⁷

• Certain strings of plain text in regular GELLMU that are part of legacy practice under LaTeX.

  1. "--"
     A short dash as used with a range of numbers, e.g., 1–2. Equivalent: \rdash.

  2. "---"
     A long dash as used for punctuation, e.g., a dash — like this. Equivalent: \pdash.

  3. "\ "
     Blank interword space. Equivalent: \spc;

  4. "\,"
     Small horizontal space. Equivalent: \hsp;

  5. "\\"
     A forced line break. “\\” may be used at the end of a line of input for a forced line break. In a tabular environment (with the didactic article document type, as in LaTeX) it begins a new tabular row. Any other use is deprecated, and will result in translation to the defined-empty element bsl corresponding to the ASCII character ‘\’ with a warning from the syntactic translator. The use of tabular is deprecated and is not supported beyond “lrc”. Use table, which is not similar to LaTeX's table, instead of tabular. Equivalents: \brk; for a line break outside of a tabular environment.⁸

  6. A blank line.
     Begin new paragraph command. Equivalent: \parb. Nearly equivalent: \par.

  7. "``"
     Left (double) quotation mark. Equivalent: \ldq.

  8. "''"
     Right (double) quotation mark. Equivalent: \rdq.

  9. "\("
     Begin math mode command. Equivalent: \begin{math}.

  10. "\)"
      End math mode command. Equivalent: \end{math}.

  11. "\["
      Begin displaymath mode command. Equivalent: \begin{displaymath}.

  12. "\]"
      End displaymath mode command. Equivalent: \end{displaymath}.

• \begin{name} . . . \end{name}

  This usage is equivalent to "\name . . . \name:", which, in turn, is equivalent to "\name{ ... }".

5.2.  Multiple Argument/Option Syntax

An essential point in the present design is that the whole system is built from components, each of which has its own function⁹. Consistent with this design the syntactic translator operates with knowledge of syntax but little or no knowledge of language.

Multiple argument/option syntax has been built into advanced mode as part of the overall idea of providing, where sensible, LaTeX-like features in a precise user markup interface for writing in document types under SGML and XML.

What are the rules for converting the multiple argument/option syntax in source markup into SGML? Direct conversion by the syntactic translator of this type of usage into XML is not available because such conversion requires some language knowledge and the program does not operate with knowledge of language at that level. One obtains an XML version of a document in the prototype production system by using a translator with minimal knowledge of the command vocabulary to create the XML version from an SGML version that is the immediate output of the syntactic translator.

In multiple argument/option syntax, which is much like that of LaTeX, arguments and options follow command names. Arguments are delimited by braces, i.e., ‘{’ and ‘}’ and options by square brackets, i.e., ‘[’ and ‘]’. There must be no white space between the arguments and options nor between the command name and the first member of an argument/option sequence.

Each command with a multiple argument/option sequence is translated to an open tag whose name is the name of the command. Each argument is translated to an ag0 element and each option to an op0 element lying in GELLMU's reserved name space. There are two exceptional cases.

1. The first argument or option is an option inside which the very first character is a colon, i.e., ‘:’. This is the method provided in advanced mode for the direct entry of an SGML attribute sequence. The entire contents of the option string, apart from the leading ‘:’, which is discarded, are understood to be a sequence of SGML attributes for the SGML element whose name is the name of the command. There is no syntax check of the attribute contents by the syntactic translator. Such an attribute option is not treated as an op0 element. In particular, an attribute option is correctly followed immediately by a semi-colon, i.e., the character ‘;’, if and only if the corresponding SGML element is a defined-empty element under the SGML document type. Since SGML attributes correspond to very little of classical LaTeX, attribute options may be entirely ignored.

2. The first argument is the only argument and there are no options apart from a possible attribute option. This case, which is extremely common, is exceptional relative to argument/option handling since the sole argument simply becomes element content without an ag0 wrapper.

5.3.  Mathematics

The Greek letter $\Gamma$ is marked, up as in LaTeX, with

The didactic article document type provides mathematical markup that is similar to that of LaTeX. For example, one may use the markup

to speak of the congruence $$2^{p-1}\equiv 1\left(mod{p}^2\right)\text{.}$$ In this example the markup “\eos” is a formal end-of-sentence¹⁰.

In a LaTeX math environment, input text “abc” is set the same as the the input text “a b c” following the presumption that each glyph is a separate symbol. At one point I had planned to provide for a distinction between these two forms of input for article. However, I am concerned about author confusion; so there will be provision for having “abc” be a math symbol, but an instance will be invoked as “\abc”.

In math mode all symbols are assumed to be single-characters as in LaTeX. A \mathsym command (part of language design that, for the moment, has no extant implementation) in the document preamble may be used to specify that a string is to be regarded as a single mathematical entity whenever it occurs in math mode. This contrasts with LaTeX in that there is no formal declaration of math symbols.

mathsym will be a variant of newcommand, which is a meta-command that is handled entirely by the syntactic translator with no trace in the output. However, the design of mathsym, which will also be a meta-command but which will leave marks in the output, is not yet decided. The design issues surround how to provide the author who is its user with fexible open means of planting semantic information in the output without undue verbosity. The syntax might be simply

Omitting the option could make author usage equivalent to that of newcommand, but still trace information planted in the output of the syntactic translator could be sufficient that in the future a derived MathML object or a derived XML formatted object produced by translation and viewed in Mozilla might enable the reader to ascertain the symbol-name and then launch a search for other occurrences. The option would serve as a mechanism for the author to pass semantic enrichment information to downstream formatters.

5.4.  Examples

The following table refers to the didatic article document type. It demonstrates how source is translated by the GELLMU Syntactic Translator to the SGML version of article and from there is translated using sgmlspl to the XML version of article.

One may find many other examples of GELLMU markup in the project archive.