previously they where included in the middle of tree-mutator.hpp
This was straight forward, since the builder relies on the classes
defined in the detail headers.
However, the GenNode-binding needs to use a specifically configured
collection binding, and this in turn requires writing a recursive
lambda to deal with nested scopes. This gets us into trouble with
circular definition dependencies.
As a workaround we now only *declare* the DSL builder functions
in the tree-mutator-builder object, and additionally use auto on
all return types. This allows us to spell out the complete builder
definition, without mentioning any of the implementation classes.
Obviously, the detail headers have then to be included *after*
the builder definition, at bottom of tree-mutator.hpp
This also allows us to turn these implementation headers into
completely normal headers, with namespaces and transitive #includes
In the end, the whole setup looks much more "innocent" now.
But beware: the #include of the implementation headers at bottom
of tree-mutator.hpp needs to be given in reverse dependency order,
due to the circular inclusion (back to tree-mutator.hpp) in
conjunction with the inclusion guards!
...instead of using a hand written implementation,
the idea is to rely on the now implemented building blocks,
with just some custom closures to make it work.
