eventfilter takes a single argument which must be a quoted string. It is true if the argument string is a substring of the Event field of the game.
event "1.p" not event "@"
The earliest correct (tested by the
not event "@" above, using the conventions of the Heijden database) 1st prize study is from 1881.
The event can also be searched for from the command line:
cql -event 1.p foo.cql