When using docinfo.internalDTD.iterentities() content is empty

I am trying to process a GDML file i.e. XML and access the !ENTITY definitions.

I can process the file by using resolve_entities=True but my application needs to access the definitions as I wish to make this information editable for saving as a new GDML file.

The start of the GDML file looks like

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE gdml [
 <!ENTITY materials SYSTEM "materialsOptical.xml">
 <!ENTITY solids_Mainz_v2 SYSTEM "solids_Mainz_v2.xml">
 <!ENTITY matrices_Mainz_v2 SYSTEM "matrices_Mainz_v2.xml">
]>

My attempt todate looks like

from lxml import etree

    myparser = etree.XMLParser(resolve_entities=False,attribute_defaults=False)
    tree = etree.parse(filename, parser=myparser)

    print(dir(tree.docinfo))
    print(tree.docinfo.doctype)
    print(dir(tree.docinfo.doctype))

    for e in tree.docinfo.internalDTD.iterentities() :
        print(e.name)
        print(e.content)
        print(e.orig)
        print(dir(e))

Which gets to print out the !ENTITY names materials, solids and matrices but I cannot see how to access their definition i.e. SYSTEM "materialsOptical.xml" etc.

The print of iterentities has
materials
None
None
['__class__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'content', 'name', 'orig']

Should the definitions not be available as content?

Thanks

python reportLXML.py
Python : sys.version_info(major=3, minor=6, micro=6, releaselevel='final', serial=0)
lxml.etree : (4, 3, 3, 0)
libxml used : (2, 9, 9)
libxml compiled : (2, 9, 9)
libxslt used : (1, 1, 33)
libxslt compiled : (1, 1, 33)