class_docstr: A little class to represent an interior node in the tree
  
  This is analogous to SymPy.Basic for non-Atoms
code: def __str__(self):
      return "%s[%s]" % (str(self.op), ", ".join(map(str, self.args)))
  
imports: from sympy.utilities.iterables import kbins
label_desc: Use image node_id 4 for calling the Compound obj's underlying member method code with example usage: obj.__str__() and returns: unknown