lambdapi fails when inputing a metavariable

[fblanqui]

symbol a:A;
symbol f : A → A → A;
symbol i:A → A;

symbol lem : A ≔
begin
apply f { print ?1 } { };
end;
// [/home/blanqui/src/lambdapi/tmp/zo.lp:8:16-18] Unknown object ?1.