  
  [1X2 [33X[0;0YHow To Type a [5XGAPDoc[105X[101X[1X Document[133X[101X
  
  [33X[0;0YIn  this chapter we give a more formal description of what you need to start
  to  type  documentation  in  [5XGAPDoc[105X  XML  format.  Many details were already
  explained by example in Section [14X1.2[114X of the introduction.[133X
  
  [33X[0;0YWe  do  [13Xnot[113X  answer  the  question  [21XHow  to [13Xwrite[113X a [5XGAPDoc[105X document?[121X in this
  chapter. You can (hopefully) find an answer to this question by studying the
  example in the introduction, see [14X1.2[114X, and learning about more details in the
  reference Chapter [14X3[114X.[133X
  
  [33X[0;0YThe definite source for all details of the official XML standard with useful
  annotations is:[133X
  
  [33X[0;0Y[7Xhttps://www.xml.com/axml/axml.html[107X[133X
  
  [33X[0;0YAlthough  this  document  must  be  quite technical, it is surprisingly well
  readable.[133X

  [1X2.1 [33X[0;0YGeneral XML Syntax[133X[101X
  
  [33X[0;0YWe  will  now  discuss  the  pieces of text which can occur in a general XML
  document.  We  start with those pieces which do not contribute to the actual
  content of the document.[133X

  [1X2.1-1 [33X[0;0YHead of XML Document[133X[101X
  
  [33X[0;0YEach XML document should have a head which states that it is an XML document
  in some encoding and which XML-defined language is used. In case of a [5XGAPDoc[105X
  document this should always look as in the following example.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[28X<?xml version="1.0" encoding="UTF-8"?>[128X[104X
    [4X[28X<!DOCTYPE Book SYSTEM "gapdoc.dtd">[128X[104X
  [4X[32X[104X
  
  [33X[0;0YSee [14X2.1-13[114X for a remark on the [21Xencoding[121X statement.[133X
  
  [33X[0;0Y(There  may  be  local  entity definitions inside the [10XDOCTYPE[110X statement, see
  Subsection [14X2.2-3[114X below.)[133X

  [1X2.1-2 [33X[0;0YComments[133X[101X
  
  [33X[0;0YA  [21Xcomment[121X  in XML starts with the character sequence [21X[10X[110X[121X. Between these sequences there must not be two adjacent dashes
  [21X[10X--[110X[121X.[133X

  [1X2.1-3 [33X[0;0YProcessing Instructions[133X[101X
  
  [33X[0;0YA  [21Xprocessing  instruction[121X  in  XML  starts  with  the character sequence [21X[10X<?[110X[121X
  followed  by  a  name  ([21X[10Xxml[110X[121X  is  only  allowed  at the very beginning of the
  document  to  declare  it  being an XML document, see [14X2.1-1[114X). After that any
  characters  may  follow,  except  that the ending sequence [21X[10X?>[110X[121X must not occur
  within the processing instruction.[133X
  
  [33X[0;0Y [133X
  
  [33X[0;0YAnd  now  we  turn  to  those  parts of the document which contribute to its
  actual content.[133X

  [1X2.1-4 [33X[0;0YNames in XML and Whitespace[133X[101X
  
  [33X[0;0YA  [21Xname[121X  in XML (used for element and attribute identifiers, see below) must
  start  with  a letter (in the encoding of the document) or with a colon [21X[10X:[110X[121X or
  underscore  [21X[10X_[110X[121X character. The following characters may also be digits, dots [21X[10X.[110X[121X
  or dashes [21X[10X-[110X[121X.[133X
  
  [33X[0;0YThis  is  a  simplified  description of the rules in the standard, which are
  concerned with lots of unicode ranges to specify what a [21Xletter[121X is.[133X
  
  [33X[0;0YSequences  only  consisting  of  the  following characters are considered as
  [13Xwhitespace[113X:   blanks,   tabs,   carriage  return  characters  and  new  line
  characters.[133X

  [1X2.1-5 [33X[0;0YElements[133X[101X
  
  [33X[0;0YThe  actual  content of an XML document consists of [21Xelements[121X. An element has
  some  [21Xcontent[121X  with  a  leading  [21Xstart  tag[121X  ([14X2.1-6[114X)  and a trailing [21Xend tag[121X
  ([14X2.1-7[114X).  The content can contain further elements but they must be properly
  nested.  One  can  define  elements  whose  content  is  always empty, those
  elements can also be entered with a single combined tag ([14X2.1-8[114X).[133X

  [1X2.1-6 [33X[0;0YStart Tags[133X[101X
  
  [33X[0;0YA  [21Xstart-tag[121X  consists of a less-than-character [21X[10X<[110X[121X directly followed (without
  whitespace)  by  an  element name (see [14X2.1-4[114X), optional attributes, optional
  whitespace, and a greater-than-character [21X[10X>[110X[121X.[133X
  
  [33X[0;0YAn  [21Xattribute[121X  consists  of some whitespace and then its name followed by an
  equal  sign  [21X[10X=[110X[121X which is optionally enclosed by whitespace, and the attribute
  value,  which  is  enclosed either in single or double quotes. The attribute
  value may not contain the type of quote used as a delimiter or the character
  [21X[10X<[110X[121X,  the  character  [21X[10X&[110X[121X  may  only  appear  to  start an entity, see [14X2.1-9[114X. We
  describe in [14X2.1-11[114X how to enter special characters in attribute values.[133X
  
  [33X[0;0YNote  especially  that  no  whitespace  is  allowed  between  the starting [21X[10X<[110X[121X
  character  and the element name. The quotes around an attribute value cannot
  be omitted. The names of elements and attributes are [13Xcase sensitive[113X.[133X

  [1X2.1-7 [33X[0;0YEnd Tags[133X[101X
  
  [33X[0;0YAn  [21Xend  tag[121X  consists  of  the  two  characters [21X[10X</[110X[121X directly followed by the
  element name, optional whitespace and a greater-than-character [21X[10X>[110X[121X.[133X

  [1X2.1-8 [33X[0;0YCombined Tags for Empty Elements[133X[101X
  
  [33X[0;0YElements  which  always have empty content can be written with a single tag.
  This   looks   like  a  start  tag  (see [14X2.1-6[114X)  [13Xexcept[113X  that  the  trailing
  greater-than-character [21X[10X>[110X[121X is substituted by the two character sequence [21X[10X/>[110X[121X.[133X

  [1X2.1-9 [33X[0;0YEntities[133X[101X
  
  [33X[0;0YAn  [21Xentity[121X in XML is a macro for some substitution text. There are two types
  of entities.[133X
  
  [33X[0;0YA  [21Xcharacter entity[121X can be used to specify characters in the encoding of the
  document  (can  be useful for entering non-ASCII characters which you cannot
  manage  to  type in directly). They are entered with a sequence [21X[10X&#[110X[121X, directly
  followed  by either some decimal digits or an [21X[10Xx[110X[121X and some hexadecimal digits,
  directly  followed  by  a semicolon [21X[10X;[110X[121X. Using such a character entity is just
  equivalent to typing the corresponding character directly.[133X
  
  [33X[0;0YThen  there  are  references  to  [21Xnamed  entities[121X.  They are entered with an
  ampersand character [21X[10X&[110X[121X directly followed by a name which is directly followed
  by  a  semicolon  [21X[10X;[110X[121X.  Such  entities  must be declared somewhere by giving a
  substitution text. This text is included in the document and the document is
  parsed  again  afterwards. The exact rules are a bit subtle but you probably
  want  to  use  this only in simple cases. Predefined entities for [5XGAPDoc[105X are
  described in [14X2.1-10[114X and [14X2.2-3[114X.[133X

  [1X2.1-10 [33X[0;0YSpecial Characters in XML[133X[101X
  
  [33X[0;0YWe  have  seen  that the less-than-character [21X[10X<[110X[121X and the ampersand character [21X[10X&[110X[121X
  start  a  tag  or  entity reference in XML. To get these characters into the
  document  text  one  has  to use entity references, namely [21X[10X<[110X[121X to get [21X[10X<[110X[121X and
  [21X[10X&[110X[121X  to  get [21X[10X&[110X[121X. Furthermore [21X[10X>[110X[121X must be used to get [21X[10X>[110X[121X when the string [21X[10X]]>[110X[121X
  appears  in  element  content  (and  not  as  delimiter  of  a [10XCDATA[110X section
  explained below).[133X
  
  [33X[0;0YAnother possibility is to use a [10XCDATA[110X statement explained in [14X2.1-12[114X.[133X

  [1X2.1-11 [33X[0;0YRules for Attribute Values[133X[101X
  
  [33X[0;0YAttribute values can contain entities which are substituted recursively. But
  except  for the entities < or a character entity it is not allowed that a
  <  character  is introduced by the substitution (there is no XML parsing for
  evaluating the attribute value, just entity substitutions).[133X

  [1X2.1-12 [33X[0;0Y[10XCDATA[110X[101X[1X[133X[101X
  
  [33X[0;0YPieces  of text which contain many characters which can be misinterpreted as
  markup  can  be  enclosed  by  the  character  sequences  [21X[10X<![CDATA[[110X[121X and [21X[10X]]>[110X[121X.
  Everything  between these sequences is considered as content of the document
  and  is  not further interpreted as XML text. All the rules explained so far
  in  this  section  do  [13Xnot  apply[113X  to  such a part of the document. The only
  document  content  which cannot be entered directly inside a [10XCDATA[110X statement
  is  the  sequence  [21X[10X]]>[110X[121X.  This  can  be  entered  as [21X[10X]]>[110X[121X outside the [10XCDATA[110X
  statement.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[28XA nesting of tags like     is not allowed.[128X[104X
  [4X[32X[104X

  [1X2.1-13 [33X[0;0YEncoding of an XML Document[133X[101X
  
  [33X[0;0YWe  suggest  to use the UTF-8 encoding for writing [5XGAPDoc[105X XML documents. But
  the  tools  described  in  Chapter  [14X5[114X  also  work  with ASCII or the various
  ISO-8859-X  encodings  (ISO-8859-1  is  also  called  latin1 and covers most
  special characters for western European languages).[133X

  [1X2.1-14 [33X[0;0YWell Formed and Valid XML Documents[133X[101X
  
  [33X[0;0YWe  want  to mention two further important words which are often used in the
  context of XML documents. A piece of text becomes a [21Xwell formed[121X XML document
  if all the formal rules described in this section are fulfilled.[133X
  
  [33X[0;0YBut  this  says  nothing  about  the  content  of the document. To give this
  content  a  meaning one needs a declaration of the element and corresponding
  attribute  names as well as of named entities which are allowed. Furthermore
  there  may  be restrictions how such elements can be nested. This [13Xdefinition
  of  an  XML  based markup language[113X is done in a [21Xdocument type definition[121X. An
  XML  document  which  contains only elements and entities declared in such a
  document  type  definition  and  obeys the rules given there is called [21Xvalid
  (with respect to this document type definition)[121X.[133X
  
  [33X[0;0YThe  main  file  of  the  [5XGAPDoc[105X package is [11Xgapdoc.dtd[111X. This contains such a
  definition  of  a  markup  language.  We are not going to explain the formal
  syntax rules for document type definitions in this section. But in Chapter [14X3[114X
  we will explain enough about it to understand the file [11Xgapdoc.dtd[111X and so the
  markup language defined there.[133X

  [1X2.2 [33X[0;0YEntering [5XGAPDoc[105X[101X[1X Documents[133X[101X
  
  [33X[0;0YHere are some additional rules for writing [5XGAPDoc[105X XML documents.[133X

  [1X2.2-1 [33X[0;0YOther special characters[133X[101X
  
  [33X[0;0YAs  [5XGAPDoc[105X  documents  are  used  to  produce  LaTeX and HTML documents, the
  question arises how to deal with characters with a special meaning for other
  applications  (for  example  [21X[10X&[110X[121X,  [21X[10X#[110X[121X,  [21X[10X$[110X[121X,  [21X[10X%[110X[121X,  [21X[10X~[110X[121X,  [21X[10X\[110X[121X, [21X[10X{[110X[121X, [21X[10X}[110X[121X, [21X[10X_[110X[121X, [21X[10X^[110X[121X, [21X[10X [110X[121X (this is a
  non-breakable  space, [21X[10X~[110X[121X in LaTeX) have a special meaning for LaTeX and [21X[10X&[110X[121X, [21X[10X<[110X[121X,
  [21X[10X>[110X[121X  have a special meaning for HTML (and XML). In [5XGAPDoc[105X you can usually just
  type  these  characters  directly,  it is the task of the converter programs
  which  translate  to  some  output  format  to  take  care  of  such special
  characters. The exceptions to this simple rule are:[133X
  
  [30X    [33X[0;6Y& and < must be entered as [10X&[110X and [10X<[110X as explained in [14X2.1-10[114X.[133X
  
  [30X    [33X[0;6YThe  content of the [5XGAPDoc[105X elements [10X<M>[110X, [10X<Math>[110X and [10X<Display>[110X is LaTeX
        code, see [14X3.8[114X.[133X
  
  [30X    [33X[0;6YThe  content of an [10X<Alt>[110X element with [10XOnly[110X attribute contains code for
        the specified output type, see [14X3.9-1[114X.[133X
  
  [33X[0;0YRemark:  In former versions of [5XGAPDoc[105X one had to use particular entities for
  all  the  special  characters  mentioned  above  ([10X&tamp;[110X,  [10X&hash;[110X, [10X$[110X,
  [10X&percent;[110X, [10X˜[110X, [10X&bslash;[110X, [10X&obrace;[110X, [10X&cbrace;[110X, [10X&uscore;[110X, [10X&circum;[110X, [10X&tlt;[110X,
  [10X&tgt;[110X). These are no longer needed, but they are still defined for backwards
  compatibility with older [5XGAPDoc[105X documents.[133X

  [1X2.2-2 [33X[0;0YMathematical Formulae[133X[101X
  
  [33X[0;0YMathematical  formulae  in  [5XGAPDoc[105X  are  typed as in LaTeX. They must be the
  content of one of three types of [5XGAPDoc[105X elements concerned with mathematical
  formulae:  [21X[10XMath[110X[121X,  [21X[10XDisplay[110X[121X,  and  [21X[10XM[110X[121X  (see  Sections [14X3.8-1[114X  and [14X3.8-2[114X for more
  details).  The  first  two  correspond to LaTeX's math mode and display math
  mode.  The last one is a special form of the [21X[10XMath[110X[121X element type, that imposes
  certain  restrictions  on the content. On the other hand the content of an [21X[10XM[110X[121X
  element is processed in a well defined way for text terminal or HTML output.
  The [21X[10XDisplay[110X[121X element also has an attribute such that its content is processed
  as in [21X[10XM[110X[121X elements.[133X
  
  [33X[0;0YNote  that  the  content  of  these  element  is LaTeX code, but the special
  characters  [21X[10X<[110X[121X  and  [21X[10X&[110X[121X  for  XML  must  be entered via the entities described
  in [14X2.1-10[114X or by using a [10XCDATA[110X statement, see [14X2.1-12[114X.[133X

  [1X2.2-3 [33X[0;0YMore Entities[133X[101X
  
  [33X[0;0YIn [5XGAPDoc[105X there are some more predefined entities:[133X
  
      ┌─────────────┬─────────┐
      │ [10X&GAP;[110X       │ [5XGAP[105X     │ 
      ├─────────────┼─────────┤
      │ [10X&GAPDoc;[110X    │ [5XGAPDoc[105X  │ 
      ├─────────────┼─────────┤
      │ [10X&TeX;[110X       │ TeX     │ 
      ├─────────────┼─────────┤
      │ [10X&LaTeX;[110X     │ LaTeX   │ 
      ├─────────────┼─────────┤
      │ [10X&BibTeX;[110X    │ BibTeX  │ 
      ├─────────────┼─────────┤
      │ [10X&MeatAxe;[110X   │ [5XMeatAxe[105X │ 
      ├─────────────┼─────────┤
      │ [10X&XGAP;[110X      │ [5XXGAP[105X    │ 
      ├─────────────┼─────────┤
      │ [10X&copyright;[110X │ ©       │ 
      ├─────────────┼─────────┤
      │ [10X&nbsp;[110X      │ [21X [121X     │ 
      ├─────────────┼─────────┤
      │ [10X&ndash;[110X     │ –       │ 
      └─────────────┴─────────┘
  
       [1XTable:[101X Predefined Entities in the [5XGAPDoc[105X system
  
  
  [33X[0;0YHere [10X [110X is a non-breakable space character.[133X
  
  [33X[0;0YAdditional  entities  are defined for some mathematical symbols, see [14X3.8[114X for
  more details.[133X
  
  [33X[0;0YOne can define further local entities right inside the head (see [14X2.1-1[114X) of a
  [5XGAPDoc[105X XML document as in the following example.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[28X<?xml version="1.0" encoding="UTF-8"?>[128X[104X
    [4X[28X[128X[104X
    [4X[28X<!DOCTYPE Book SYSTEM "gapdoc.dtd"[128X[104X
    [4X[28X  [ <!ENTITY MyEntity "some longish <E>text</E> possibly with markup">[128X[104X
    [4X[28X  ]>[128X[104X
  [4X[32X[104X
  
  [33X[0;0YThese  additional  definitions go into the [10X<!DOCTYPE[110X tag in square brackets.
  Such new entities are used like this: [10X&MyEntity;[110X[133X
  
