Factorial can be computed using an iterative way : let result be 1, then for each value V between 1 and N, multiply result by V.
Memory: N 0 0 0 0 0
Cursor: first cell
set result to 1
set current index to 0
invariant: result = (current index)! and first cell + current index = N
while first cell is not null
multiply result by current index
decrease first cell
first cell = 0, so current index = N and result = N!
>>>>+ set result to 1 (and current index to 0)
<<<<[ while first cell is not null
>[->+>+<<]>[-<+>]> copy current index
[->[->+>+<<]>[-<+>]<<] multiply result by current index
>>>[-<<+>>]<<<<<+ increase current index and store result x new index into result
<-] decrease first cell and loop