are double cells safe to use?

[Roland Orre]

I consider using double cells for efficient implementation of binary trees.

Is this safe? (i.e. no pre-assumptions made somewhere what these are used for)

For prototyping I intended to add
double-cons
set-cbr!
set-ccr!
cbr
ccr

to the scheme level. Is this safe?
They will only contain or refer to SCM data types.

Of course I could add an own datatype, but as double cells contains
exactly what I need and they are used internally I consider them being
the obvious choice.

I wrote to both user and devel as I consider this may be more of a
devel question.