how to traverse a parse tree (list)

do_atom(a)
  switch typeof(a)
    NUM:
    GLOBAL:
    LOCAL:

go(e)
  if e == nil stop
  if isatom(e) do_atom(e)
  //  then e is a list  (operator operand*)
  a = first(e)
  b = tail(e)
  switch typeof(a)
    PLUS: go(first(b)) + go(second(b))  
    MINUS:
    ...
  

how to create a parse tree

example: copy a tree

copy(a) 
  if isatom(a)  return newatom(copyof(a))
  else return cons(copy(first(a)), copy(tail(a)) )