class_docstr: Symmetric mean absolute percentage error.
  
  Examples
  --------
  
  >>> from river import metrics
  
  >>> y_true = [0, 0.07533, 0.07533, 0.07533, 0.07533, 0.07533, 0.07533, 0.0672, 0.0672]
  >>> y_pred = [0, 0.102, 0.107, 0.047, 0.1, 0.032, 0.047, 0.108, 0.089]
  
  >>> metric = metrics.SMAPE()
  >>> for yt, yp in zip(y_true, y_pred):
  ...     metric.update(yt, yp)
  
  >>> metric
  SMAPE: 37.869392
code: def _eval(self, y_true, y_pred):
      den = abs(y_true) + abs(y_pred)
      if den == 0:
          return 0.0
      return 2.0 * abs(y_true - y_pred) / den
  
imports: from __future__ import annotations
  from river import metrics
label_desc: Use image node_id 1 for calling the SMAPE obj's underlying member method code with example usage: obj._eval(y_true, y_pred) and returns: unknown, int