ShortestUniquePrefix←{
⍝ ⍵: Nested list of words 
⍝ ←: Shortest unique prefixes, not in order
     I←((⊃⊣)⌷⊢)⍤0 99            ⍝ Select indexer
     ⍺←1                        ⍝ Prefix length     
     p←⍺∘↑¨⍵                    ⍝ Prefixes
     (u c i)←↓⍉{(⍺)(≢⍵)(⍵)}⌸p   ⍝ (Unique Count Indices)
     um←c=1                     ⍝ Unique prefix mask
     0=⍴⍵:⍬                     ⍝ All SUPs found: Return empty array
     0=∨/um:(⍺+1)∇ ⍵            ⍝ No unique prefixes: 
                                ⍝   increment prefix length
                                ⍝     ∇ recurse
     cwi←i⌿⍨um                  ⍝ Completed word indices
     cw←⍵ I⍨cwi                 ⍝ Completed words
     up←u⌿⍨um                   ⍝ Unique prefixes
                                ⍝ Unique prefixes found: 
                                ⍝   Return unique prefixes,
                                ⍝     increment prefix length
                                ⍝       ∇ recurse with remaining words
     ∨⌿um:(⊃¨up),(⍺+1)∇ ⍵~cw 
 }