sub evalRna {
    my ($l, $r) = @_;
    my %bonds    = (GU => 1, UG => 1, AU => 2, UA => 2, CG => 3, GC => 3);
    my $lu = "$bases[$l]$bases[$r]";
    my $numBonds = $bonds{$lu} || die "Bad bond type '$lu' given $l, $r\n";

##</code><code>##

Bad bond type '5C' given 0, 21