How to register sub printers?

[g199209]

My ./gdbinit:

python
import sys
sys.path.insert(1, '/home/xxx/Boost-Pretty-Printer')
import boost
boost.register_printers()
end

$gdb
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*** boost: registering top-level printers: (name="boost" id=140709847321104) (name="trivial" id=140709847321168) with objfile=None
(gdb) info pretty-printer
global pretty-printers:
  boost
  builtin
    mpx_bound128
  trivial

Only top level boost pretty-printer is registered, no any sub printers.

I have tried the example program in README, the pretty-printer has no effect:

(gdb) p range
$1 = {<boost::iterator_range_detail::iterator_range_base<char const*, boost::iterators::random_access_traversal_tag>> = {<boost::iterator_range_detail::iterator_range_base<char const*, boost::iterators::bidirectional_traversal_tag>> = {<boost::iterator_range_detail::iterator_range_base<char const*, boost::iterators::incrementable_traversal_tag>> = {<boost::iterator_range_detail::iterator_range_tag> = {<No data fields>}, m_Begin = 0x7fffffffde24 "Hello World",
        m_End = 0x7fffffffde30 ""}, <No data fields>}, <No data fields>}, <No data fields>}

BTW, when I tried to specified boost version in .gdbinit like this:

boost.register_printers(boost_version=(1,72,0))

Python error occured:

Traceback (most recent call last):
  File "<string>", line 4, in <module>
TypeError: register_printers() got an unexpected keyword argument 'boost_version'
/home/xxx/.gdbinit:16: Error in sourced command file:
Error while executing Python code.

Is README doc out of date? How to use this printer correctly now?