RelativePosition operator, represented by "+" in WCCL (at least initially). Returns Nowhere for Nowhere, or a Position shifted by given offset to the position given (Begin and End are first translated into normal Positions).

Claryfing comment for linux antlr lib (it should only be included for final execs not for intermediate libs)

Change StrSet::is_subset_of to correctly handle empty sets, add some comments
Add Tset::insert_symbol
Add tests for new functions
Bump required corpus2 version to 0.1.2 due to a fixed bug there

Do not link antlr lib on linux. Need to investigate error and why does it look like working without the linkage. It's still needed for win tho.

LogicalPredicate, abstract base class for predicates that are logical functions taking any number of arguments

Add PositionRef value class, add Position:: nowhere/begin/end, add SentenceContext get_[abs|rel]_position takint Position or Positionref
Add tests for sc's get_*_position