def valid? *indexes
      indexes.all? { |i| to_a.include?(i) || (i.is_a?(Numeric) && i < size) }
    end
Returns true if all arguments are either a valid category or position
@param indexes [Array<object>] categories or positions
@return [true, false]
@example
idx.valid? :a, 2
# => true
idx.valid? 3
# => false