mod DUPLICATE is
    inc LOOP-MODE .
    op null : -> State .
    vars Input Output : QidList .
    crl [duplicate] : 
       [Input, null, Output] 
       => [nil, null, Output Input Input] 
          if Input =/= nil .
  endm

  loop [nil, null, nil] .

  (a s d )

  *** a s d a s d