Queries are passed to the command line, using the keywords OR
and NOT. Grouping is done using [ and ] (these are not reserved by
the shell). Two tildes (`~~`) at the beginning of a string denote
a glob match on tags. A tilde and a dash (`~-`) denote a glob match
on titles.

The grammar is implemented as a recursive descent parser.

```
e1 ::= e2 "OR" e1 | e2;
e2 ::= e3 e2 | e3;
e3 ::= "NOT" e4 | e4;
e4 ::= TAG | "[" e1 "]";
```