at contains those definitions.
In most use cases, fftrees_define passes a new FFTrees object x either
to fftrees_grow_fan (to create new FFTs by applying algorithms to data) or
to fftrees_wordstofftrees (if my.tree is specified).
If an existing FFTrees object object or tree.definitions are provided as inputs,
no new FFTs are created.
When both arguments are provided, tree.definitions take priority over the FFTs in an existing object.
Specifically,

If tree.definitions are provided, these are assigned to the