Extract a set of reference contexts for each variant in the collection.

    Parameters
    ----------
    variants : varcode.VariantCollection

    context_size : int
        Max of nucleotides to include to the left and right of the variant
        in the context sequence.

    transcript_id_whitelist : set, optional
        If given, then only consider transcripts whose IDs are in this set.

    Returns a dictionary from variants to lists of ReferenceContext objects,
    sorted by max coding sequence length of any transcript.

def reference_contexts_for_variants(
        variants,
        context_size,
        transcript_id_whitelist=None):
    """
    Extract a set of reference contexts for each variant in the collection.

    Parameters
    ----------
    variants : varcode.VariantCollection

    context_size : int
        Max of nucleotides to include to the left and right of the variant
        in the context sequence.

    transcript_id_whitelist : set, optional
        If given, then only consider transcripts whose IDs are in this set.

    Returns a dictionary from variants to lists of ReferenceContext objects,
    sorted by max coding sequence length of any transcript.
    """
    result = OrderedDict()
    for variant in variants:
        result[variant] = reference_contexts_for_variant(
            variant=variant,
            context_size=context_size,
            transcript_id_whitelist=transcript_id_whitelist)
    return result