def normalize(ast)
      if ast.class == SExpSymbol
        case ast.name
        when "nil"
          return SExpNil.new
        else
          return ast
        end
      elsif ast.cons? then
        ast.visit do |i|
          normalize(i)
        end
      end
      return ast
    end
def normalize(ast)
      if ast.class == SExpSymbol
        case ast.name
        when "nil"
          return SExpNil.new
        else
          return ast
        end
      elsif ast.cons? then
        ast.visit do |i|
          normalize(i)
        end
      end
      return ast
    end
replace special symbols