This is the double Theme, Stretta, Diminution, Inverse Fugue!
bar1-23 : Theme 1
bar24-36 : Theme 1(Original + Diminution) Stretta
bar37-54 : Theme 1 + 2, Double Fugue
bar54-87 : Theme 1(Orignal + Diminution & Inverse) + Theme 2(Original & Inverse), Double Stretta & Inverse Fugue