bit string version of property_tree. The ram resident structure of the ptree is
mostly compatible with one created using the xml_parser from
boost::property_tree, with slight differences in the way atoms are stored. The
result is you can use Format() to convert from xml to s-expression, but not the
other way around. You can write a simple s-expression beautifier in just a few
lines of code.

The main value however is the s-expression parser, i.e. Scan(), which is an
alternative to crafting a custom recursive descent parser for a particular
grammar. The tipping point depends on whether you want to read only a small
portion of a much larger document. If so, then using the ptree will likely be a
"faster to code" route. Documentation on how to navigate a ptree can be found on
the boost website and there are a number of examples in the
pcbnew/eagle_plugin.cpp file in this project. Powerful path navigation support
makes it easy to extract a subset of a ptree.