code: def _theta_transform(Z, trend, theta):
      # obtain one Theta-line
      theta_line = Z * theta + (1 - theta) * trend
      theta_line = theta_line.values.flatten()
      return theta_line
  
file_path: sktime/sktime/transformations/series/theta.py
imports: import numpy
  import pandas
  from sktime.forecasting.base import ForecastingHorizon
  from sktime.forecasting.trend import PolynomialTrendForecaster
  from sktime.transformations.base import BaseTransformer
label_desc: Use image node_id 1 for calling a global function with example usage: _theta_transform(Z, trend, theta) and returns: theta_line