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Key to Notation
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Throughout this manual, (in Sections 1 and 3 in particular,)
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the  syntax  of  commands  is  described  through the use of
various 'meta-symbols'.  These symbols comprise a system  of
notation  commonly  known  as  'Backus-Naur Form', or simply
BNF.  What follows is a brief description of the BNF that is
used in this documentation.
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<>\A word or phrase enclosed in left and right  angle
brackets stands for any string of characters whose
meaning  is either suggested by the word or phrase
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so enclosed or explicitly defined  later  in  the
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syntax.   For  example, "<number>" might stand for
"127" or "3" or "98.6".  Words or phrases enclosed
in  these  brackets  are  called  'meta-linguistic
variables'.
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::=\This  symbol  means "is defined as" and it is used
to separate a meta-linguistic  variable  from  its
definition.  For example,
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.be
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<number> ::= <integer>
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would be read "a number is defined as an integer."
Everything  to  the right of the "::=" is called a
'meta-linguistic formula'.
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|\The  vertical  bar  means  "or"  and  is  used  to
separate  alternatives  within  a  meta-linguistic
formula.  For example,
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.be
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<number> ::= <integer> | <real>
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.ee
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would be read "a number is defined as  an  integer
or a real."
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()\Parentheses are used to enclose a series of alter-
natives  in  a  formula  when the series comprises
only one part of the formula.  For example,
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.be
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<signed_number> ::= (+|-)<number>
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.ee
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would be read "a signed number  is  defined  as  a
plus sign or a minus sign, followed by a number."
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[]\Formulae  (or  parts thereof) that are enclosed in
square brackets are optional.  For example,
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<command> ::= <filename> [<parameters>]
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.ee
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would be read "a command is defined as a filename,
optionally followed by parameters."
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{}\Formulae that are enclosed in curly braces may  be
repeated any number of times, including zero.  For
example,
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.be
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<integer> ::= <digit>{<digit>}
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.ee
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would  be  read  "an integer is defined as a digit
followed by zero or more digits."
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In situations where the syntax  requires  that  one  of  the
above  meta-symbols appear literally, the symbol is enclosed
in apostrophes.  For example, in
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.be
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<vertical_bar> ::= '|'
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.ee
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the vertical bar on the right hand side is interpreted as  a
literal character, not as an "or" symbol.