def witness_commitment
      return nil if !op_return? || op_return_data.bytesize < 36
      buf = StringIO.new(op_return_data)
      return nil unless buf.read(4).bth == WITNESS_COMMITMENT_HEADER
      buf.read(32).bth
    end
def witness_commitment
      return nil if !op_return? || op_return_data.bytesize < 36
      buf = StringIO.new(op_return_data)
      return nil unless buf.read(4).bth == WITNESS_COMMITMENT_HEADER
      buf.read(32).bth
    end
get witness commitment