Inproper decision for '<=>' and '>=<' operators

[KES777]

The FULL join is combination of RIGHT <= and LEFT => joins.
Thus FULL must be: <=>.

The second ambiguous fact is that FULL is >=<.
The FULL join is OUTER join, same as RIGHT and LEFT. OUTER!

1. Thus for OUTER arrows must be directed to out too: <=>

2. For INNER join they must be directed inside: >=<