#ExtractCycle(pi,i): The cycle corresponding to the member i of the permutation pi For example ExtractCycle([2,1,4,3],1)=[1,2]
ExtractCycle:=proc(pi,i): local C, ng: C:=[i]: ng:=pi[i]: while ng<>i do C:=[op(C),ng]: ng:=pi[ng]: end do; C; end proc:

CycDec:=proc(pi): local n, i, StillToDo, S, C, ng: n:=nops(pi): StillToDo:={seq(i,i=1..n)}: S:={}: while StillToDo<>{} do ng:=StillToDo[1]:
C:=ExtractCycle(pi,ng): S:=S union {C}: StillToDo:=StillToDo minus {op(C)}: end do; S; end proc: