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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 passthrough_repl(self, inputstring, **kwargs):
 """Add back passthroughs."""

        out = []
        index = None
        for c in append_it(inputstring, None):
            try:

                if index is not None:
                    if c is not None and c in nums:
                        index += c
                    elif c == unwrapper and index:
                        ref = self.get_ref("passthrough", index)
                        out.append(ref)
                        index = None
                    elif c != "\\" or index:
                        out.append("\\" + index)
                        if c is not None:
                            out.append(c)
                        index = None
                elif c is not None:
                    if c == "\\":
                        index = ""
                    else:
                        out.append(c)

            except CoconutInternalException as err:
                complain(err)
                if index is not None:
                    out.append(index)
                    index = None
                out.append(c)

        return "".join(out)