4-level stack manipulation on 12c

[Gene]

Tony Hutchins gave this 7-step solution without using LASTX.

ENTER
ENTER
CLx
-
Roll Down
X<>Y
Roll Down

But more importantly, the PDF where this and a large number of such ideas is found here:

Stack Rearrangements

You can find this solution by searching for ABAB, which represents XYXY as the contents of the four level RPN stack.