here's a pattern to try:
first do 6 checkerboards of order 2 (F2 B2 R2 L2 U2 D2) and then do
superfliptwist. in other words, the group product of these two elements.

Was certainly one of the hardest to do.  After 17 hours the best was 22
turns, but then results came in, after 18 hours 21 turns, and finally
after 19 hours 20 turns:
  F1 R1 L2 U3 R2 L3 U3 D2 R2 F1 D1 B1 D1 F2 U3 R3 D3 F2 D2 L2
This on an SGI R4K Indigo.

dik
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dik t. winter, cwi, kruislaan 413, 1098 sj amsterdam, nederland
home: bovenover 215, 1025 jn amsterdam, nederland; e-mail: dik@cwi.nl