class_docstr: Base class for mystic functions
  
  The 'function' method must be overwritten, thus allowing
  calls to the class instance to mimic calls to the function object.
  
  For example, if function is overwritten with the Rosenbrock function:
      >>> rosen = Rosenbrock(ndim=3)
      >>> rosen([1,1,1])
      0.
     
code: def __call__(self, *args, **kwds):
      coeffs = (
          kwds["coeffs"]
          if "coeffs" in kwds
          else (args[0] if len(args) else [])
      )
      if len(coeffs) != self.ndim:
          raise ValueError(
              "input length does not match ndim (ndim=%d)" % self.ndim
          )
      return self.function(*args, **kwds)
  
file_path: mystic/mystic/models/abstract_model.py
imports: from numpy import sum
  from mystic.forward_model import CostFactory
label_desc: Use image node_id 2 for calling the AbstractFunction obj's underlying member method code with example usage: obj.__call__() and returns: self