The `NAT`

tag is used to wrap implementation
(in our case Pico, 'real' Pico) functions such that they can be used by MC-eval.
A `NAT`

tree is simply a tag together with the associated native function.
Whenever `nn`

is a tree representing such a primitive function, then
the function can be retrieved using `nn[NAT_NAT_idx]`

. The name can
be accessed through `nn[NAT_NAM_idx]`

. Of course, in our case where
primitive functions our implemented in Pico, this name will be the same af the
name of the function in the underlying Pico. In the C version, this is not necessarily
the case.