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- Applied changes necessary to migrate hosting of pyparsing source
over to GitHub. Many thanks for help and contributions from hugovk,
jdufresne, and cngkaygusuz among others through this transition,
sorry it took me so long!

- Fixed import of collections.abc to address DeprecationWarnings
in Python 3.7.

- Updated oc.py example to support function calls in arithmetic
expressions; fixed regex for '==' operator; and added packrat
parsing. Raised on the pyparsing wiki by Boris Marin, thanks!

- Fixed bug in select_parser.py example, group_by_terms was not
reported. Reported on SF bugs by Adam Groszer, thanks Adam!

- Added "Getting Started" section to the module docstring, to 
guide new users to the most common starting points in pyparsing's
API.

- Fixed bug in Literal and Keyword classes, which erroneously
raised IndexError instead of ParseException.