new_contents: # Strategies to traverse a SymPy Tree
  
  def top_down(rule):
      def top_down_traversal(expr):
          expr = rule(expr)
          return expr.__class__(*map(rule, expr.args))
      return top_down_traversal
  
  def bottom_up(rule):
      def bottom_up_traversal(expr):
          return rule(expr.__class__(*map(rule, expr.args)))
      return bottom_up
  
subject: add basic traversals