U2 was made by Orozco since 11 years ago, but in this moment there were inneficient algorithm.
The idea behind U2 is to use it as a "Base method", with UF/UFR as the buffers, the mechanism is the same than M2 and R2,.
Corners (Speffz)
A: U2
B: [l' R' : [D2, R U R']] + U2
C: Buffer
D: [R' D R...