list$ = "ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB CDBA CBAD ABDC ADBC BDCA DCBA BACD BADC BDAC CBDA DBCA DCAB"

for a = asc("A") to asc("D")
  for b = asc("A") to asc("D")
    for c = asc("A") to asc("D")
      for d = asc("A") to asc("D")
        x$ = chr$(a) + chr$(b) + chr$(c)+ chr$(d)
        for i = 1 to 4                                            ' make sure each letter is unique
          j = instr(x$,mid$(x$,i,1))
          if instr(x$,mid$(x$,i,1),j + 1) <> 0 then goto [nxt]
        next i
       if instr(list$,x$) = 0 then print x$;" missing"            ' found missing permutation
[nxt] next d
    next c
  next b
next a