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gives(ovid,money,irs).
gives(ovid,book,grep).
gives(ovid,book,kudra).

?- gives(ovid,book,kudra).

?- gives(ovid,book,irs).

?- give(ovid,book,Who).
Who=grep
Who=kudra

{
  'gives' => {
...
      }
   }
};

my @recipients = keys %{$facts{gives}{Ovid}{book}};

if ( exists $facts{gives}{Ovid}{book}{merlyn} ) { ... }

?- gives(Who,book,_);
Who=ovid
Who=kudra
yes

my @givers;
foreach my $person ( keys %{$facts{gives}} ) {
    push @givers => $person if exists $facts{ gives }{ $person }{ book
+ };
}

gives(ovid,book(learning_perl), merlyn).

?- gives(ovid, book(Title), merlyn).
Title=learning_perl

gives(kudra,book(learning_perl),merlyn).

location( kitchen_drawer, [ knife, fork, kitten ] ).

positive(Number) :-
    Number > 0.