Use of uninitialized value in subroutine entry at Levels/Level_IV.pm line 437.
Can't use string ("") as a subroutine ref while "strict refs" in use at Levels/Level_IV.pm line 437.

##</code><code>##

Undefined subroutine &Levels::Level_IV::_dydt called at /usr/lib/perl5/site_perl/5.8.8/Math/RungeKutta.pm line 74.