class_docstr: A generic container for immutable data that can be accessed either by
  numeric index or by key. This is similar to an
  :class:`collections.OrderedDict` except that the keys are optional and
  iteration over it returns the values instead of keys.
  
  This is the base class for both :class:`.Column` and :class:`.Row`.
  
  :param values:
      A sequence of values.
  :param keys:
      A sequence of keys.
code: def __init__(self, values, keys=None):
      self._values = tuple(values)
  
      if keys is not None:
          self._keys = keys
      else:
          self._keys = None
  
imports: from collections import OrderedDict
  from collections.abc import Sequence
  from agate.utils import memoize
label_desc: Use image node_id 1 to create a new MappedSequence object from inherited base classes: Sequence with example: obj = MappedSequence(values, keys)