I.E. empty sentence shouldn't be passed anyway and that is what
Operator class is checking for. But otherwise, at the level of
operator, it can deal with a sentence that has current pos outside
of boundaries.

It is called "flex" atm but it is subject to change.

Also, remove redundant goto_starts

It is more intuitive that way, e.g. equal(2,3) is never true, equal(3,3) is always true.
Nowhere is only equal to Nowhere, and Begin or End are equal to nonspecial positions
depending on current position in the sentence context.

* reset_values function to set all Values to their defaul-construct state
* read-only access to all variables of some type via get_all<T>
* Variables cloning
* convenience functions: get_or_throw, get_value
Value subtypes now contain a value_type typedef and require a const value_type& get_value() const member function.

It's a change from if x then y else z, which wasn't exactly proper anyway without parentheses.

So tools like valgrind don't get too excited that there's still reachable but not freed memory.