> So far I have noticed the following.
 > * In l3expan the following definitions are missing
 >    \def_new:Npn \exp_args:NNnx {\::N\::n\::x\:::}
 >    \def_new:Npn \exp_args:NNox {\::N\::o\::x\:::}
 >    \def_new:Npn \exp_args:NNoo {\::N\::o\::o\:::}

good point. that module needs a bit mor standardization anyway to have all
typical 1-5 arg expansions predefined

 > * The docu of l3prop says that \prop_map_inline refers to the
 >   arguments via ##1 and ##2. But it should be #1 and #2.


 > * What is \prop_gput:Ooo for? Hasn't got \prop_gput:Noo the
 >   same functionality? (Let alone that \prop_gput:Ooo doesn't
 >   work, yet.)

O in contrast to o means single expanion on an input which is a single token
ie no braces around it.

\tlp_new:N \l_tmp_tlp
\prop_new:N \g_foo_prop

\tlp_set:Nn \l_tmp_tlp {\g_foo_prop}

\prop_gput:Ooo \l_tmp_tlp ...

don't ask me why that is predefined. perhaps i needed it for the xor module at
one stage ... it is pretty weird and i don't think there is much use in
setting it up unless there is actually a use for it.

and then there is the question that you say it doesn't work ... perhaps i
better look at that one.