I genuinely cannot stop thinking about this. both times. BOTH TIMES, the Doctor calmed down thinking of this. A poem with a direct reference to Missy, to the Master.
Because it's not a poem outside of the Doctor Who is it? Correct me if I'm wrong but google isn't too helpful anymore. Its something tailor made. That is calming, or at least distracting, to the Doctor on such a deep level.
Did the Doctor make the poem? An embellished memory of losing the moon and stealing the president's daughter?
Or perhaps its the version that became popular with the Shabogans. Rumours turned into rhyme. A real tale kept alive by a touch of fiction. Told and spread orally, eventually making its way back to the Doctor.
Or was it already a story. The Doctor narrowly avoiding recreating it (as they tend to do). Missy referencing the old poem in her fib. A now a private joke between her and the Doctor.
But more so, it feels like a conversation. The Doctor and the Master talking before they left Gallifrey. The Doctor causing all sorts of havoc but still hesitating to actually leave. The Master questioning them: Is this all there is for you? Didn't we agree there was more?
Whatever it is its making me FERAL