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

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.

Xplor-NIH 2023-11-10