class_docstr: A ``SequenceField`` represents a sequence of things.  This class just adds a method onto
  ``Field``: :func:`sequence_length`.  It exists so that ``SequenceLabelField``, ``IndexField`` and other
  similar ``Fields`` can have a single type to require, with a consistent API, whether they are
  pointing to words in a ``TextField``, items in a ``ListField``, or something else.
class_docstr_tok: ["A","``","SequenceField","``","represents","a","sequence","of","things",".","This","class","just","adds","a","method","onto","``","Field","``",":",":","func",":","`","sequence_length","`",".","It","exists","so","that","``","SequenceLabelField","``",",","``","IndexField","``","and","other","similar","``","Fields","``","can","have","a","single","type","to","require",",","with","a","consistent","API",",","whether","they","are","pointing","to","words","in","a","``","TextField","``",",","items","in","a","``","ListField","``",",","or","something","else","."]
docstr: How many elements are there in this sequence?
docstr_tok: ["How","many","elements","are","there","in","this","sequence","?"]
code: def sequence_length(self):
      """
      How many elements are there in this sequence?
      """
      raise NotImplementedError
  
imports: from __future__ import absolute_import
  from allennlp.data.fields.field import Field
label_desc: Use image node_id 1 for calling the SequenceField obj's underlying member method code with example usage: obj.sequence_length() without return types