Expressions

An OPM Query Language expression may be either a variable, a primitive value or a finite set of primitive values.

<expression> ::= <variable>
               | <primitive value>
               | <set of values>
               ;

<primitive value> ::= <number>
                   | <string>
                   ;

<set of values> ::= '{' <set of numbers> '}'
                  | '{' <set of strings> '}'
                  ;
<set of numbers> ::= <number>
                   | <set of numbers> ',' <number>
                   ;
<set of strings> ::= <string>
                   | <set of strings> ',' <string>
                   ;

Given an instantiation of variables, we may extend it to assign values to expressions: a variable expression is assigned the same value as associated with the variable by the instantiation, while values are assigned to primitive expressions (numbers and strings) in the standard way.