Basic XQuery primitive.
The following construct(s) refer to this construct:
A PrimaryExpr is composed of the language primitives of
XQuery. These include variables, literals, function calls, and parenthesized
expressions.
Variables consist of a "$" prefix character
and a variable name, which must be a QName. Literals are divided
into string literals and some forms of numeric literals. An
ElementNameOrFunctionCall either represents a simple element name,
which is a QName or a call of a function defined in Tamino XQuery
4. Wildcard expressions and KindTest are used in the context of
path expressions, while an expression that is enclosed in parentheses
influences the precedence of the operators.
Some literal expressions
42 42.0 "42" 42E2
The first literal denotes the integer value 42, the second literal denotes the decimal value 42.0. The third literal is a string literal that is composed of the characters "4" and "2", while the last literal denotes a value of double precision.
Variables are used in FLWOR and FLWU expressions such as the following:
for $a in input()/bib/book where $a/@year >= 2000 return $a/title
