Some people might enjoy a C311 Interpreter in Co-dfns. This was done at the request of Jason Hemann.

Interpret←{⍺←0 2⍴⍬ ⋄ t←⊃0 1⌷⍵
  t≡'Lambda':⍵
  t≡'Number':⊃⊢/⍵
  t≡'Symbol':⊃((0⌷⍉⍺)⍳0 2⌷⍵)1⌷⍉⍺
  ⊃((⍺⍪⍨(0 2⌷⊣),∘⊂⊢)∇1↓⊣)/⍺∘∇¨⍵⊂[0]⍨(1+⊃⍵)=0⌷⍉⍵
}

This version uses a flat encoding of the program tree similiar to the ⎕XML format.

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