docs: def print_tree(self, ast_obj=None):
          """Convert AST object to tree view of BEL AST
  
          Returns:
              prints tree of BEL AST to STDOUT
          """
  
          if not ast_obj:
              ast_obj = self
  
          if hasattr(self, "bel_subject"):
              print("Subject:")
              self.bel_subject.print_tree(self.bel_subject, indent=0)
  
          if hasattr(self, "bel_relation"):
              print("Relation:", self.bel_relation)
  
          if hasattr(self, "bel_object"):
              if self.bel_object.type == "BELAst":
                  if hasattr(self, "bel_subject"):
                      print("Nested Subject:")
                      self.bel_object.bel_subject.print_tree(indent=0)
  
                  if hasattr(self, "bel_relation"):
                      print("Nested Relation:", self.bel_object.bel_relation)
  
                  if hasattr(self, "bel_object"):
                      print("Nested Object:")
                      self.bel_object.bel_object.print_tree(indent=0)
              else:
                  print("Object:")
                  self.bel_object.print_tree(self.bel_object, indent=0)
  
          return self