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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_fields(a, b):
 """Merge two lists of fields. Fields in `b` override fields in `a`. Fields in `a` are output first. """


    a_names = set(x[0] for x in a)
    b_names = set(x[0] for x in b)
    a_keep = a_names - b_names

    fields = []

    for name, field in a:
        if name in a_keep:
            fields.append((name, field))

    fields.extend(b)

    return fields