code: def _pairs(keys, *, start=1, sep=' AND '):
      '''
      >>> _pairs(['foo', 'bar', 'baz'], sep=', ')
      'foo=$1, bar=$2, baz=$3'
      >>> _pairs(['foo', 'bar', 'baz'], start=2)
      'foo=$2 AND bar=$3 AND baz=$4'
      '''
      return sep.join('{}=${}'.format(k, i) for i, k in enumerate(keys, start)f _pairs(keys, *, start=1, sep=' AND '):
      '''
      >>> _pairs(['foo', 'bar', 'baz'], sep=', ')
      'foo=$1, bar=$2, baz=$3'
      >>> _pairs(['foo', 'bar', 'baz'], start=2)
      'foo=$2 AND bar=$3 AND baz=$4'
      '''
      return sep.join('{}=${}'.format(k, i) for i, k in enumerate(keys, start))
docs: >>> _pairs(['foo', 'bar', 'baz'], sep=', ')
      'foo=$1, bar=$2, baz=$3'
      >>> _pairs(['foo', 'bar', 'baz'], start=2)
      'foo=$2 AND bar=$3 AND baz=$4'