Ajd chapter10

Chapter 10
I did a little test to confirm my suspicion - that's not how autovivification works.  We'd need to be checking for definedness of a key at a lower level of nesting for the level above it to be autovivified.

e.g. `if( defined $exit->{foo}->{$guess} )` would autovivify $exit->{foo} if it didn't exist, so that the interpreter can check what's inside it, but `$exit->{foo}->{$guess}` would not be created by this check.