<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_parents_letters(self, goobj):
 """Get the letters representing all parent terms which are depth-01 GO terms."""

        parents_all = set.union(self.go2parents[goobj.id])
        parents_all.add(goobj.id)
        # print "{}({}) D{:02}".format(goobj.id, goobj.name, goobj.depth), parents_all
        parents_d1 = parents_all.intersection(self.gos_depth1)
        return [self.goone2ntletter[g].D1 for g in parents_d1]