How to use the uproot.write.sink.cursor.Cursor function in uproot

def __setitem__(self, where, what):
        self.util = Util()
        where, cycle = self._normalizewhere(where)
        if what.__class__.__name__ != "TTree" and what.__class__.__name__ != "newtree":
            what = uproot_methods.convert.towriteable(what)
        elif what.__class__.__name__ == "newtree":
            what = TTree(where, what, self)
        cursor = uproot.write.sink.cursor.Cursor(self._fSeekFree)
        newkey = uproot.write.TKey.TKey(fClassName = what._fClassName,
                                        fName      = where,
                                        fTitle     = what._fTitle,
                                        fObjlen    = 0,
                                        fSeekKey   = self._fSeekFree,
                                        fSeekPdir  = self._fBEGIN,
                                        fCycle     = cycle if cycle is not None else self._rootdir.newcycle(where))
        if what.__class__.__name__ == "newtree" or what.__class__.__name__ == "TTree":
            # Need to (re)attach the cycle number to allow getitem to access writable TTree
            tree_where = where + b";" + str(newkey.fCycle).encode("utf-8")
            self._treedict[tree_where] = what
        newkeycursor = uproot.write.sink.cursor.Cursor(newkey.fSeekKey)
        newkey.write(cursor, self._sink)
        what._write(self, cursor, where, self.compression, newkey, newkeycursor, self.util)
        self._expandfile(cursor)

def _writestreamers(self):
        self._fSeekInfo = self._fSeekFree
        self._seekcursor.update_fields(self._sink, self._format_seekinfo, self._fSeekInfo)

        cursor = uproot.write.sink.cursor.Cursor(self._fSeekInfo)
        streamerkey = uproot.write.TKey.TKey32(fClassName = b"TList",
                                               fName      = b"StreamerInfo",
                                               fTitle     = b"Doubly linked list",
                                               fSeekKey   = self._fSeekInfo,
                                               fSeekPdir  = self._fBEGIN)
        streamerkeycursor = uproot.write.sink.cursor.Cursor(self._fSeekInfo)
        streamerkey.write(cursor, self._sink)

        uproot.write.compress.write(self, cursor, uproot.write.streamers.streamers, self.compression, streamerkey, streamerkeycursor)

        self._fNbytesInfo = streamerkey.fNbytes
        self._nbytescursor.update_fields(self._sink, self._format_nbytesinfo, self._fNbytesInfo)

        self._expandfile(cursor)

def _expandfile(self, cursor):
        if cursor.index > self._fSeekFree:
            freecursor = uproot.write.sink.cursor.Cursor(cursor.index)
            freekey = uproot.write.TKey.TKey(b"TFile", self._filename, fObjlen=0, fSeekKey=cursor.index, fSeekPdir=self._fBEGIN)
            freeseg = uproot.write.TFree.TFree(cursor.index + freekey.fNbytes)
            freekey.fObjlen = freeseg.size()
            freekey.fNbytes += freekey.fObjlen

            freekey.write(freecursor, self._sink)
            freeseg.write(freecursor, self._sink)

            self._fSeekFree = cursor.index
            self._fEND = cursor.index + freekey.fNbytes
            self._fNbytesFree = freekey.fNbytes
            self._nfree = 1
            self._endcursor.update_fields(self._sink, self._format_end, self._fEND, self._fSeekFree, self._fNbytesFree, self._nfree)

def fKeylen(self):
        return self._format1.size + uproot.write.sink.cursor.Cursor.length_strings([self.fClassName, self.fName, self.fTitle])

self._endcursor = uproot.write.sink.cursor.Cursor(cursor.index)
        cursor.write_fields(self._sink, self._format_end, self._fEND, self._fSeekFree, self._fNbytesFree, self._nfree)

        self._fNbytesName = 2*len(self._filename) + 36 + 8   # + 8 because two fields in TKey are 'q' rather than 'i'
        fUnits = 4
        cursor.write_fields(self._sink, self._format2, self._fNbytesName, fUnits)

        self._compresscursor = uproot.write.sink.cursor.Cursor(cursor.index)
        cursor.write_fields(self._sink, self._format3, self._fCompress)

        self._fSeekInfo = 0
        self._seekcursor = uproot.write.sink.cursor.Cursor(cursor.index)
        cursor.write_fields(self._sink, self._format_seekinfo, self._fSeekInfo)

        self._fNbytesInfo = 0
        self._nbytescursor = uproot.write.sink.cursor.Cursor(cursor.index)
        cursor.write_fields(self._sink, self._format_nbytesinfo, self._fNbytesInfo)

        cursor.write_data(self._sink, b'\x00\x01' + uuid.uuid1().bytes)

def writekeys(self, cursor):
        self.fSeekKeys = cursor.index
        self.fNbytesKeys = self._nbyteskeys()

        self.tfile._expandfile(uproot.write.sink.cursor.Cursor(self.fSeekKeys + self.allocationbytes))

        self.keycursor = uproot.write.sink.cursor.Cursor(self.fSeekKeys)
        self.headkey.write(self.keycursor, self.sink)
        self.nkeycursor = uproot.write.sink.cursor.Cursor(self.keycursor.index)
        self.keycursor.write_fields(self.sink, self._format2, len(self.keys))
        for key in self.keys.values():
            key.write(self.keycursor, self.sink)

        self.update()

def writekeys(self, cursor):
        self.fSeekKeys = cursor.index
        self.fNbytesKeys = self._nbyteskeys()

        self.tfile._expandfile(uproot.write.sink.cursor.Cursor(self.fSeekKeys + self.allocationbytes))

        self.keycursor = uproot.write.sink.cursor.Cursor(self.fSeekKeys)
        self.headkey.write(self.keycursor, self.sink)
        self.nkeycursor = uproot.write.sink.cursor.Cursor(self.keycursor.index)
        self.keycursor.write_fields(self.sink, self._format2, len(self.keys))
        for key in self.keys.values():
            key.write(self.keycursor, self.sink)

        self.update()

def writekeys(self, cursor):
        self.fSeekKeys = cursor.index
        self.fNbytesKeys = self._nbyteskeys()

        self.tfile._expandfile(uproot.write.sink.cursor.Cursor(self.fSeekKeys + self.allocationbytes))

        self.keycursor = uproot.write.sink.cursor.Cursor(self.fSeekKeys)
        self.headkey.write(self.keycursor, self.sink)
        self.nkeycursor = uproot.write.sink.cursor.Cursor(self.keycursor.index)
        self.keycursor.write_fields(self.sink, self._format2, len(self.keys))
        for key in self.keys.values():
            key.dircursor = copy(self.keycursor)
            key.write(self.keycursor, self.sink)

        self.update()