This is the ability - given a copy of txt2tags in the PYTHONPATH with a .py 
extension - to, from within a Python script, import txt2tags as a module and 
pass a list of strings (each one corresponding to a line) and to receive as a 
return value a similar list representing the output file. There seems to have 
already been some work done working towards this, but (at least as far as I can 
tell) it isn't quite there yet.
This was quite simple for me to fix - there were two functions, 
"process_source_file" and "convert_this_files", which accepted an optional 
parameter specifying the contents of a file. This parameter simply wasn't 
passed in by the "exec_command_line" and "get_infiles_config" functions. 
Though this patch did pass the tests to the same degree as r643 does (flying 
colors except the sample module complains about the aap target and the art 
module complains about the aat target) and it does work with one "virtual" 
file, I would be very uncertain about attempting to use multiple such files.
Also, to avoid the Python misfeature of mutable default arguments of a function 
not being emptied with multiple calls to said function, both default arguments 
to "exec_command_line" are now tuples instead of lists, though they are 
immediately converted to lists.