Notation

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 $\vert$. Use of the $\vert$ metasymbol is somewhat loose; it is left out where alternatives are specified on different printed lines, e.g.,

$<$dummy-statement$>$:== A $\vert$ B $\vert$ C

is equivalent to

$<$dummy-statement$>$:==

A

text

B

text

C

text

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    end

is optional; i.e.,

set    message on    end

is 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.