code: def _find_free_name(self, prefix):
      idx = 0
      while True:
          name = prefix + str(idx)
          found = False
          for v in self.functions.values():
              if name in v.graph:
                  found = True
                  break
          if found:
              idx += 1
          else:
              return name
  
imports: import copy
  from coremltools import _logger
  from coremltools.converters.mil.mil import types
  from .basic_graph_ops import check_connections, const_determined_nodes
  from .dot_visitor import DotVisitor
  from .naming_utils import escape_fn_name
label_desc: Use image node_id 14 for calling the NetworkEnsemble obj's underlying member method code with example usage: obj._find_free_name(prefix) and returns: name