- parenthesis are applied to clauses via a new _Grouping construct.

- parenthesis are applied to clauses via a new _Grouping construct.
uses operator precedence to more intelligently apply parenthesis
to clauses, provides cleaner nesting of clauses (doesnt mutate
clauses placed in other clauses, i.e. no 'parens' flag)
- added 'modifier' keyword, works like func.<foo> except does not
add parenthesis.  e.g. select([modifier.DISTINCT(...)]) etc.