def index_of element
      case dtype
      when :array then @index.key(@data.index { |x| x.eql? element })
      else @index.key @data.index(element)
      end
    end
def index_of element
      case dtype
      when :array then @index.key(@data.index { |x| x.eql? element })
      else @index.key @data.index(element)
      end
    end
Get index of element