<Filter-exp> ::= <Term> | <Term><Prox-op><Term> | <Exist-op><<Term> | <Filter-exp><Bool-op><Filter-exp>
<Term> ::= string | <Term> [<Wild>]
<Prox-op> ::= nearby(Number) | ...
<Existop> ::= + | -
<Bool-op> ::= and | or
<Wild> ::= *
<Rank-exp> ::= <RTerm> | <Rank-exp><Bool-op><Rank-exp>
<RTerm> ::= <Term>Gewicht | <RTerm>Gewicht | <Meta>(<RTerm>)Gewicht
<Meta> ::= author | title | ...