Test for relabel. They will fail since there's no way of writing incomplete tags. To consider and either fix or remove the feature.

Note: one of the tests fails on purpose so everyone can see what it looks like.

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.