VowelStrings←{
⍝ ⍺: String sequence rules
     ⍺←('aei' 'ei' 'iaeou' 'oau' 'uoe') ⍝ Default
⍝ 'a' can only be followed by 'e' and 'i'
⍝ 'e' can only be followed by 'i'
⍝ 'i' can only be followed by 'a', 'e', 'o' and 'u'
⍝ 'o' can only be followed by 'a' and 'u'
⍝ 'u' can only be followed by 'o' and 'e'
     (chars rules)←(⊃¨⍺)(⍺)
⍝ ⍵: String size (1≤⍵)∧(⍵≤≢⍺)
⍝ ←: All rule-obeying strings of length ⍵
     Next←{⊃,/(⊆∘.,1↓rules⊃⍨chars⍳(⊃⌽))¨⊆⍵}   ⍝ Rule-following superstring of length (1+≢⍵)
     superstrings←Next⍣(⍵-1)¨chars            ⍝ Length ⍵ superstrings of each char
     ⍪↑⊃,/superstrings                        ⍝ Return flat matrix
⍝ One-liner: ⍪↑⊃,/{⊃,/(⊆∘.,1↓rules⊃⍨chars⍳(⊃⌽))¨⊆⍵}⍣(⍵-1)¨chars
 }