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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 _flds_append(flds, addthese, dont_add):
 """Retain order of fields as we add them once to the list."""

        for fld in addthese:
            if fld not in flds and fld not in dont_add:
                flds.append(fld)