It's the "setvar($V, blabla)" operator that sets $V to blabla and always returns True.

Somewhat nicer to use and guards against static init order fiasco.

It is used wherever in WCCL variable is seen in read-only context, i.e. outside of operators that are setting its value like setvar.

Generally we want to avoid accidental change of value of a constant or of a variable. Originally I'd copy value on access, but this changed it so a const Value is returned instead and copy is done only when it's needed.

Functions operate on it now instead of on just SentenceContext. It's open to additions in future. Main purpose atm is to hold Variables as well as SentenceContext.
Done some cleanup in code too. Updated tests to reflect the FunExecContext change as well.

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

Conditional operators realised as if..then..else expression and operator "? val ? pred" with some tests for StrSet specialization

add a VariableAccesor class in Variables and corresponding create_accesor and get_fast functions in Variables, plus tests

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