def as_tag(value)
      case true
      when value.tag?
        element(value) ? value : nil
      when value.dicom_name?
        get_tag(value)
      when value.dicom_method?
        get_tag(@names_from_methods[value.to_sym])
      else
        nil
      end
    end
def as_tag(value)
      case true
      when value.tag?
        element(value) ? value : nil
      when value.dicom_name?
        get_tag(value)
      when value.dicom_method?
        get_tag(@names_from_methods[value.to_sym])
      else
        nil
      end
    end
Gives the tag corresponding to the specified element string value.

 @param [String] value an element tag, element name or an element's method name
 @return [String, NilClass] the matched element tag, or nil if no match is made