Throughout this book, the Backus-Naur notation (Naur 1960) is used.
Syntactic constructs are denoted by English words enclosed by
angle brackets < and >; the possibility of a wildcard specification
is indicated by enclosing the syntactic construct in
<* *> angle
brackets (Section 2.6). A
definition of a new syntactic construct is indicated by the metasymbol
``:==".
Possible repetition of a syntactic construct is
indicated by enclosing the construct within metabraces { and
}. Please note that these braces ({,}) are also
used by X-PLOR for comments. Optional (i.e., not always necessary)
constructs are enclosed
in square brackets 
and 
. Alternate constructs are separated
by the metasymbol |. Use of the | metasymbol is somewhat
loose; it is left out where alternatives are specified
on different printed lines, e.g.,
Words or identifiers not enclosed in angle brackets should be typed as specified. Generally, uppercase letters are mandatory, whereas lowercase letters are optional. The equal sign ``=" that is used in many assignments, e.g.,
set message=on endis optional; i.e.,
set message on endis a valid statement. However, in mathematical expressions (see Sections 2.14 and 2.16), the equal sign is mandatory.
Throughout this book, many actual example inputs for X-PLOR will be provided. To distinguish them from the explanatory text, they have been typeset in ``typewriter" font with consecutive line numbering. The pointer
{===>}
indicates lines of the file that most likely need modification.