Path Expressions

We first extend our projection elements to include reverse attributes and attributes with implicit classes. The new BNF for projection elements is

<projection element> ::= <attribute name> '[' <class name> ']'
                       | '!' <attribute name> '[' <class name> ']'
                       | <attribute name>
                       ;

Here the construct ! attr[ Class] represents a reverse attribute. For example the variable declarations Y IN FRAGMENT, X IN Y.!owns[PERSON], would be equivalent to the declarations X IN PERSON, Y IN X.owns[FRAGMENT].

If the [ Class] is omitted from a projection element, then it is equivalent to using the class of the attribute. For example if the owns attribute of class PERSON has class FRAGMENT, then the declaration X IN Y.owns is equivalent to X IN Y.owns[FRAGMENT].

Next we allow variable declarations to involve series of attributes or reverse attributes rather than just single attributes. The extended syntax for variable declarations is

<variable declaration> ::= <variable> IN <class expression>
                         ;
<class expression> ::= <class name>
                     | <variable> '.' <projection composition>
                     ;
<projection composition> ::= <composition element list> <projection element>
                           ;
<composition element list> ::= <null>
                             | <composition element> <composition element list>
                             ;
<composition element> ::= <attribute name> '[' <class name> ']'
                        | '!' <attribute name> '[' <class name> ']'
                        | <attribute name> '.'
                        ;

A variable declaration involving a projection composition is equivalent to a series of variable declarations involving implicit or system generated variables. For example the variable declaration X IN Y.owns[FRAGMENT]sequence would be equivalent to _1 IN Y.owns[FRAGMENT], X IN _1.sequence[VARCHAR(200)], where _1 is a new variable.