clarified a comment
first implementation done.
BUG in output: words which occur in multiple articles list the same article multiple times instead of listing the different articles in which they occur
fixed test input file (according to the expected output of LemmaIndex using StringIntegerList)
added debug prints in LOG (INFO, not DEBUG, cause I don't know yet how to switch on printing the DEBUG messages)
FIXES ISSUE. Fixed the bug. Done :)