code: def _merge(self):
      if len(self._values) == 0:
          return []
      elif len(self._values) == 1:
          return to_numpy(self._values[0]).ravel()
      else:
          values = [to_numpy(v).ravel() for v in self._values]
          merged = np.concatenate(values, axis=0)
          self._values = [merged]
  
          return merged
  
imports: from abc import ABC
  from typing import Dict, Optional
  import numpy
  from .None import Indicator
  from ...util.values import to_numpy
label_desc: Use image node_id 2 for calling the _Collection obj's underlying member method code with example usage: obj._merge() and returns: list, to_numpy, merged