- [feature] Revised the rules used to determine

- [feature] Revised the rules used to determine
the operator precedence for the user-defined
operator, i.e. that granted using the ``op()``
method.   Previously, the smallest precedence
was applied in all cases, now the default
precedence is zero, lower than all operators
except "comma" (such as, used in the argument
list of a ``func`` call) and "AS", and is
also customizable via the "precedence" argument
on the ``op()`` method.  [ticket:2537]