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list ← 90 47 58 29 22 32 55 5 55 73
fd ← {⍺=0:⍵⋄(⍺-1)∇(1↓⍵)-(¯1↓⍵)}
1 fd list
¯43 11 ¯29 ¯7 10 23 ¯50 50 18
2 fd list
54 ¯40 22 17 13 ¯73 100 ¯32