def has_one(region_name, **opts, &block)
      within = opts[:in] || opts[:within]
      region_class = opts[:class] || opts[:region_class]
      define_region_accessor(region_name, within: within, region_class: region_class, &block)
    end
def has_one(region_name, **opts, &block)
      within = opts[:in] || opts[:within]
      region_class = opts[:class] || opts[:region_class]
      define_region_accessor(region_name, within: within, region_class: region_class, &block)
    end
Defines region accessor.

 @param [Symbol] region_name
 @param [Hash|Proc] in, optional, alias: within
 @param [Class] class, optional, alias: region_class
 @param [Block] block