"SyntalxTreeNode" object not iterable error about pydens HOT 2 CLOSED

'form': lambda u, t: D(u, t) - 2 * np.pi * 2 * t^3

'form': lambda u, t: D(u, t) - 2 * np.pi * 2 * t**3

Traceback (most recent call last):

  File "<ipython-input-26-1b5c74329633>", line 1, in <module>
    runfile('/home/nirvik/Documents/neuronal_model_python/temp.py', wdir='/home/nirvik/Documents/neuronal_model_python')

  File "/usr/lib/python3/dist-packages/spyder/utils/site/sitecustomize.py", line 705, in runfile
    execfile(filename, namespace)

  File "/usr/lib/python3/dist-packages/spyder/utils/site/sitecustomize.py", line 102, in execfile
    exec(compile(f.read(), filename, 'exec'), namespace)

  File "/home/nirvik/Documents/neuronal_model_python/temp.py", line 38, in <module>
    dg = Solver(config)

  File "/home/nirvik/.local/lib/python3.6/site-packages/pydens/wrapper.py", line 21, in __init__
    self.model = model_class(config)

  File "/home/nirvik/.local/lib/python3.6/site-packages/pydens/batchflow/batchflow/models/tf/base.py", line 262, in __init__
    super().__init__(*args, **kwargs)

  File "/home/nirvik/.local/lib/python3.6/site-packages/pydens/batchflow/batchflow/models/base.py", line 38, in __init__
    self.build(*args, **kwargs)

  File "/home/nirvik/.local/lib/python3.6/site-packages/pydens/batchflow/batchflow/models/tf/base.py", line 331, in build
    config = self.build_config()

  File "/home/nirvik/.local/lib/python3.6/site-packages/pydens/model_tf.py", line 117, in build_config
    n_parameters = get_num_parameters(form[0])

  File "/home/nirvik/.local/lib/python3.6/site-packages/pydens/syntax_tree.py", line 77, in get_num_parameters
    tree = form(*[SyntaxTreeNode('_' + str(i)) for i in range(n_args)])

  File "/home/nirvik/Documents/neuronal_model_python/temp.py", line 25, in <lambda>
    'form': lambda u, t: D(u, t) - 2 * np.pi * 2*t**3,

  File "/home/nirvik/.local/lib/python3.6/site-packages/pydens/tokens.py", line 63, in <lambda>
    if isinstance(args[0], SyntaxTreeNode) else method_(*args, **kwargs))

  File "/home/nirvik/.local/lib/python3.6/site-packages/pydens/letters.py", line 82, in D
    return np.stack([self.D(func, coordinate) for coordinate in coordinates],

TypeError: 'SyntaxTreeNode' object is not iterable