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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 merge_subtokens(doc, label="subtok"):
 """Merge subtokens into a single token. doc (Doc):
 The Doc object. label (unicode):
 The subtoken dependency label. RETURNS (Doc):
 The Doc object with merged subtokens. DOCS: https://spacy.io/api/pipeline-functions#merge_subtokens """

    merger = Matcher(doc.vocab)
    merger.add("SUBTOK", None, [{"DEP": label, "op": "+"}])
    matches = merger(doc)
    spans = [doc[start : end + 1] for _, start, end in matches]
    with doc.retokenize() as retokenizer:
        for span in spans:
            retokenizer.merge(span)
    return doc