<filter> := <filterdef> | ( <filter> ) | <filter> <operator> <filter>;
<filterdef>  := <modifier> <criterion> | <criterion>;
<operator> := '∪' | '∩' | '\';
<modifier> := more | less | most | least;
<criterion> := frequent | maintainable | expensive | changeable | error-prone | usage-pattern | state-variation | concurrent | latent;