I believe we need more changes of the kind made in #40. For example _35_3_Providing_the_function_to_solve.jl still has a use of Ref{Void} in a struct. This is only correct if you want the struct to contain exactly a pointer to the object nothing. @vtjnash am I right about that?

Problems arise when storing a Ptr{Void} to such a field, because Ptr{Void} is no-op convertible to Ref{Void}, so the field ends up pointing to a boxed pointer. This strikes me as a bug in the type safety of Ref.