code: def gain_from_merging(self, other_encoding):
      combined_chars = other_encoding.chars | self.chars
      combined_width = bit_count(combined_chars)
      combined_columns = self.columns | other_encoding.columns
      combined_overhead = _Encoding._characteristic_overhead(
          combined_columns
      )
      combined_gain = (
          +self.overhead
          + other_encoding.overhead
          - combined_overhead
          - (combined_width - self.width) * len(self.items)
          - (combined_width - other_encoding.width)
          * len(other_encoding.items)
      )
      return combined_gain
  
imports: from fontTools.misc.roundTools import noRound, otRound
  from fontTools.misc.intTools import bit_count
  from fontTools.ttLib.tables import otTables
  from fontTools.varLib.models import supportScalar
  from fontTools.varLib.builder import buildVarRegionList, buildVarStore, buildVarRegion, buildVarData
  from functools import partial
  from collections import defaultdict
  from heapq import heappush, heappop
label_desc: Use image node_id 10 for calling the _Encoding obj's underlying member method code with example usage: obj.gain_from_merging(other_encoding) and returns: combined_gain