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sub foo {
    bar() or die "we failed";
...
};

die "it worked" if foo();

sub foo {
    my $c = shift;
...
};

foo( sub {die 'it worked'} ) || die "we failed\";

/* frank and dean are male */
male(frank).
...
/* frank sang with judy, frank sang with dean */
sang_with(frank, judy).
sang_with(frank, dean).

use Test::More 'no_plan';

...
ok(!$v2->bind($v1),            'cannot bind bound var');
ok( $v1->bind($v2),            'can bind unbound var');
is( $v1->value, "hello",    '  to correct value');

use strict;
use warnings;
...
    my $self = shift;
    $$self = \undef;
};

sub unify {
    my ($v1, $v2, $continuation) = @_;
...
    };
    return(0);
};

male(frank).
male(dean).

sub male {
    my ($var, $continuation) = @_;
    unify("frank", $var, $continuation);
    unify("dean", $var, $continuation);
};

sub male {
    unify("frank", @_);
    unify("dean", @_);
};

# print out all the males
my $a = Var->new;
...
# is dean male
eval {male("dean", sub {Success->throw})};
print $@ ? "dean is male" : "dean is not male", "\n";

sub female {
...
    my ($var, $continuation) = @_;
    female($var, sub {acts($var, $continuation)});
};

# print out all of the actors
my $c = Var->new;
...
# is ella an actress
eval {actress("ella", sub {Success->throw})};
print $@ ? "ella is an actress" : "ella is not an actress", "\n";

/* frank sang with judy, frank sang with dean */
sang_with(frank, judy).
sang_with(frank, dean).

sub sang_with {
    my ($p1, $p2, $continuation) = @_;    
    unify($p1, 'frank', sub {unify($p2, 'judy', $continuation)});
    unify($p1, 'frank', sub {unify($p2, 'dean', $continuation)});
};

sub unify_all {
    my ($a, $b, $continuation) = @_;
...
    };
    return(0);
};

sub sang_with {
    my ($p1, $p2, $continuation) = @_;
    unify_all(['frank', 'judy'], [$p1, $p2], $continuation);
    unify_all(['frank', 'dean'], [$p1, $p2], $continuation);
};

my $x = Var->new;
eval { 
...
    )
};
print "frank did sing with an actor\n" if $@;

my $x = Var->new;
my $succeed = sub { Success->throw };
my $sang_with = sub { sang_with("frank", $x, $succeed) };
eval { actor($x, $sang_with) };
print "frank did sing with an actor\n" if $@;