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