code: def pprint_nodes(subtrees):
      """
      Prettyprints systems of nodes.
  
      Examples
      ========
  
      >>> from sympy.printing.tree import pprint_nodes
      >>> print(pprint_nodes(["a", "b1\\nb2", "c"]))
      +-a
      +-b1
      | b2
      +-c
  
      """
  
      def indent(s, type=1):
          x = s.split("\n")
          r = "+-%s\n" % x[0]
          for a in x[1:]:
              if a == "":
                  continue
              if type == 1:
                  r += "| %s\n" % a
              else:
                  r += "  %s\n" % a
          return r
  
      if not subtrees:
          return ""
      f = ""
      for a in subtrees[:-1]:
          f += indent(a)
      f += indent(subtrees[-1], 2)
      return f
  
label_desc: Use image node_id 1 for calling a global function with example usage: pprint_nodes(subtrees) and returns: f, r, str