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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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