def add_reference_context_args(parser):
    """
    Extends an ArgumentParser instance with the following commandline arguments:
        --context-size
    """
    reference_context_group = parser.add_argument_group('Reference Transcripts'
        )
    reference_context_group.add_argument('--reference-context-size', type=
        int, default=30, help=
        'Number of nucleotides used to match assembled sequence to reference transcript to establish reading frame.'
        )
    return reference_context_group
