Is nested namespace supported?

I don't find where in the R document the discussion of nested
namespace is. If there is nested namespace supported in R, could
somebody let me know whether the document is?

Could somebody let me know if there is nested name space supported?
For example, is it possible to define a function in the name space
'nested_namespace' which is nested in the name space 'some_namespace'?

some_namespace:::nested_namespace:::a_function()