sub can_cast {
    my ($rPlayer, $rCard) = @_;

    for my $manaType (keys %{$rCard->{cost}}) {
        return 0 if ! exists $rPlayer->{mana}{$manaType};
        return 0 if $rPlayer->{mana}{$manaType} < $rCard->{cost}{$manaType};
    }

    # Nothing rejected, so we can cast it!
    return 1;
}