@Section
  @Tag { lexical }
  @Title { Lexical structure (words, spaces, symbols) and macros }
@Begin
@PP
The input to Lout consists of a sequence of @I {textual units},
textual.unit @Index {Textual unit }
which may be
either {@I{white spaces}},
@I identifiers,
@I delimiters,
or
@I {literal words}.  Each
is a sequence of @I characters chosen from:
letter @Index { Letter character }
other @Index { Other character }
quote @Index { Quote character }
escape @Index { Escape character }
comment.char @Index { Comment character }
underscore.char @Index { Underscore character }
@ID @Tab
    vmargin { 0.5vx }
    @Fmta { @Col A ! @Col B }
{
@Rowa A { letter      } B { @Code "@ab-zAB-Z_" }
@Rowa A { white space } B { @I { space  formfeed  tab  newline } }
@Rowa A { quote       } B { @Code "\"" }
@Rowa A { escape      } B { @Code "\\" }
@Rowa A { comment     } B { @Code "#" }
@Rowa A { other       } B { @Code "!$%&'()*+,-./0123456789:;<=>?[]^`{|}~" }
}
Notice that @Code "@" and @Code "_" are classed as letters.  Basser
Lout accepts the accented letters of the ISO-LATIN-1 character set
(depending on how it is installed), and these are also classed as
letters.  The ten digits are classed as `other' characters, and in
fact the `other' class contains all 8-bit characters (except octal 0)
not assigned to previous classes.
@PP
A @I {white space} is a sequence of one or more white space characters.
white.space @Index { White space }
formfeed @Index { Formfeed }
space.f @Index { Space }
 Lout treats the formfeed character exactly like the space character;
it is useful for getting page breaks when printing Lout source code.
@PP
A @I delimiter is a sequence of one or more `other' characters which
delimiter @Index { Delimiter }
is the name of a symbol.  For example, @Code "{" and @Code "//" are
delimiters.  When defining a delimiter, the name must be enclosed
in quotes:
@ID @Code {
"def  \"^\"  { {}  ^&  {} }"
}
but quotes are not used when the delimiter is invoked.  A delimiter may
have delimiters and any other characters adjacent, whereas identifiers
may not be adjacent to letters or other identifiers.  The complete list
of predefined delimiters is
@ID @OneRow @Code {
{
      "/"
  @JL "//"
  @JL "^/"
  @JL "^//"
} |2.2cx {
      "|"
  @JL "||"
  @JL "^|"
  @JL "^||"
} |2.2cx {
      "&"
  @JL "^&"
} |2.2cx {
      "&&"
  @JL "{"
  @JL "}"
}
}
A longer delimiter like @Code "<=" will be recognised in
preference to a shorter one like {@Code "<"}.
@PP
An @I identifier is a sequence of one or more letters which is the name of a
identifier @Index { Identifier }
symbol.  It is conventional but not essential to begin identifiers with
{@Code "@"};  Basser Lout will print a warning message if it finds an
unquoted literal word (see below) beginning with {@Code "@"}, since such
words are usually misspelt identifiers.  The ten digits are not letters
and may not appear in identifiers; and although the underscore character
is a letter and may be used in identifiers, it is not conventional to
do so.  The complete list of predefined identifiers is
@ID @OneRow @Code {
{     "@BackEnd"
  @JL "@Background"
  @JL "@Begin"
  @JL "@BeginHeaderComponent"
  @JL "@Break"
  @JL "@Case"
  @JL "@ClearHeaderComponent"
  @JL "@Common"
  @JL "@Char"
  @JL "@CurrFace"
  @JL "@CurrFamily"
  @JL "@CurrLang"
  @JL "@CurrYUnit"
  @JL "@CurrZUnit"
  @JL "@Database"
  @JL "@End"
  @JL "@EndHeaderComponent"
  @JL "@Enclose"
  @JL "@Filter"
  @JL "@FilterErr"
  @JL "@FilterIn"
  @JL "@FilterOut"
  @JL "@Font"
  @JL "@ForceGalley"
  @JL "@Galley"
  @JL "@Graphic"
  @JL "@HAdjust"
  @JL "@HContract"
  @JL "@HCover"
  @JL "@HExpand"
  @JL "@High"
  @JL "@HLimited"
  @JL "@HMirror"
  @JL "@HScale"
  @JL "@HShift"
} |4.4cx {
      "@HSpan"
  @JL "@Include"
  @JL "@IncludeGraphic"
  @JL "@IncludeGraphicRepeated"
  @JL "@Insert"
  @JL "@KernShrink"
  @JL "@Key"
  @JL "@Language"
  @JL "@LClos"
  @JL "@LEnv"
  @JL "@LInput"
  @JL "@LVis"
  @JL "@LUse"
  @JL "@LinkSource"
  @JL "@LinkDest"
  @JL "@Meld"
  @JL "@Merge"
  @JL "@Minus"
  @JL "@Moment"
  @JL "@Next"
  @JL "@NotRevealed"
  @JL "@Null"
  @JL "@OneCol"
  @JL "@OneOf"
  @JL "@OneRow"
  @JL "@Open"
  @JL "@Optimize"
  @JL "@Outline"
  @JL "@PAdjust"
  @JL "@PageLabel"
  @JL "@PlainGraphic"
  @JL "@Plus"
  @JL "@PrependGraphic"
  @JL "@RawVerbatim"
  @JL "@Rotate"
} |4.4cx {
      "@Rump"
  @JL "@Scale"
  @JL "@SetColor"
  @JL "@SetColour"
  @JL "@SetHeaderComponent"
  @JL "@Space"
  @JL "@StartHSpan"
  @JL "@StartHVSpan"
  @JL "@StartVSpan"
  @JL "@SysDatabase"
  @JL "@SysInclude"
  @JL "@SysIncludeGraphic"
  @JL "@SysIncludeGraphicRepeated"
  @JL "@SysPrependGraphic"
  @JL "@Tag"
  @JL "@Tagged"
  @JL "@Target"
  @JL "@Texture"
  @JL "@Underline"
  @JL "@Use"
  @JL "@URLLink"
  @JL "@VAdjust"
  @JL "@VContract"
  @JL "@VCover"
  @JL "@Verbatim"
  @JL "@VExpand"
  @JL "@VLimited"
  @JL "@VMirror"
  @JL "@VScale"
  @JL "@VShift"
  @JL "@VSpan"
  @JL "@Wide"
  @JL "@Yield"
  @JL "@YUnit"
  @JL "@ZUnit"
}
}
plus the names of the parameters of @@Moment.  The symbols @@LClos, @@LEnv,
lclos @Index { @@LClos symbol }
lenv @Index { @@LEnv symbol }
linput @Index { @@LInput symbol }
lvis @Index { @@LVis symbol }
luse @Index { @@LUse symbol }
@@LInput, @@LVis and @@LUse appear in cross reference databases generated
by Lout and are not for use elsewhere.
@PP
A sequence of characters which is neither a white space, an identifier, nor a
delimiter, is by default a @I {literal word}, which means that it will
word @Index { Word }
literal.word @Index { Literal word }
quoted.word @Index { Quoted word }
pass through Lout unchanged.  An arbitrary sequence of characters
enclosed in double quotes, for example @Code "\"{  }\"", is also a
literal word.  Space characters may be included, but not tabs or
newlines.  There are special character sequences, used only between
quotes, for obtaining otherwise inaccessible characters:
@ID @Tab
   vmargin { 0.5vx }
   @Fmta { @Col A ! @Col B }
{
@Rowa A { @Code "\\\""   } B { produces @Code "\"" }
@Rowa A { @Code "\\\\"   } B { "\\" }
@Rowa A { @Code "\\ddd"  } B { the character whose ASCII code is }
@Rowa A {                } B { the up to three digit octal number {@Code ddd} }
}
So, for example, @Code "\"\\\"@PP\\\"\"" produces {@Code "\"@PP\""}.
@PP
When the comment character
comment @Index { Comment }
@Code "#" is encountered, everything from
that point to the end of the line is ignored.  This is useful for
including reminders to oneself, like this:
@ID @OneRow @Code {
"# Lout user manual"
"# J. Kingston, June 1989"
}
for temporarily deleting parts of the document, and so on.
@PP
@I Macros
macro @Index { Macro }
provide a means of defining symbols which stand for a
sequence of textual units rather than an object.  For example, the macro
definition
@ID @Code {
"macro  @PP  {  //1.3vx  2.0f @Wide  &0i }"
}
makes Lout replace the symbol @Code "@PP" by the given textual units
before assembling its input into objects.  A similar macro to this
one is used to separate the paragraphs of the present document.  The
enclosing braces and any spaces adjacent to them are dropped, which can
be a problem:  @Code "@PP2i" has result {@Code "//1.3vx 2.0f @Wide &0i2i"}
which is erroneous.
@PP
The meaning of symbols used within the body of a macro is determined by
where the macro is defined, not by where it is used.  Due to implementation
problems, @@Open symbols will not work within macros.  Named and body
parameters will work if the symbol that they are parameters of is also
present.  There is no way to get a left or right brace into the body of
a macro without the matching brace.
@PP
Macros may be nested within other definitions and exported, but they may
not be parameters.  They may not have parameters or nested definitions
of their own, and consequently a preceding @Code export clause (Section
{@NumberOf visibility}) would be pointless; however, an @Code import
clause is permitted.
@End @Section
