code: def update(self, columns=(), by=(), where=(), **kwds):
          
          return self._seu('update', columns, by, where, kwds)
docs: update from self
  
          >>> t = q('([]a:1 2 3; b:10 20 30)')
          >>> t.update('a*2',
          ...          where='b > 20').show()  # doctest: +NORMALIZE_WHITESPACE
          a b
          ----
          1 10
          2 20
          6 30