NAT tag is used to wrap implementation
(in our case Pico, 'real' Pico) functions such that they can be used by MC-eval.
NAT tree is simply a tag together with the associated native function.
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