Help my son find his letters
APL (Dyalog Classic), 21 bytes



∊⊢,⊢∘⊂~¨(,\⊣⊂⍨1,2&gt;/⍋)


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This is a train equivalent to {∊⍵,(⊂⍵)~¨(,\⍺⊂⍨1,2&gt;/⍺⍋⍵)}

⍋ gives the permutation of right argument ⍵ in left argument ⍺

1,2&gt;/ compare consecutive pairs with &gt; and prepend a 1

⍺⊂⍨ use the above boolean mask to split ⍺ into groups; 1s in the mask mark the start of a new group

,\ cumulative concatenations of the groups

(⊂⍵)~¨ complement of each with respect to ⍵

⍵, prepend ⍵

∊ flatten as a single string