code: def serialize_icon(img):
      if img.shape[2] < 4 or np.any(img[:, :, :-1] > 0):
          raise ValueError("Image must be alpha-only")
  
      alpha = img[:, :, -1]
      return etas.serialize_numpy_array(alpha)
  
imports: import itertools
  import warnings
  import numpy
  import sklearn.linear_model
  import sklearn.metrics
  import eta.core.serial
  import eta.core.utils
  import fiftyone.core.expressions
  import fiftyone.core.labels
label_desc: Use image node_id 12 for calling a global function with example usage: serialize_icon(img) and returns: etas