def value(tag)
      check_key(tag, :value)
      if exists?(tag)
        if self[tag].is_parent?
          raise ArgumentError, "Illegal parameter '#{tag}'. Parent elements, like the referenced '#{@tags[tag].class}', have no value. Only Element tags are valid."
        else
          return self[tag].value
        end
      else
        return nil
      end
    end
def value(tag)
      check_key(tag, :value)
      if exists?(tag)
        if self[tag].is_parent?
          raise ArgumentError, "Illegal parameter '#{tag}'. Parent elements, like the referenced '#{@tags[tag].class}', have no value. Only Element tags are valid."
        else
          return self[tag].value
        end
      else
        return nil
      end
    end
Gives the value of a specific Element child of this parent.

 * Only Element instances have values. Parent elements like Sequence and Item have no value themselves.
 * If the specified tag is that of a parent element, an exception is raised.

 @param [String] tag a tag string which identifies the child Element
 @return [String, Integer, Float, NilClass] an element value (or nil, if no element is matched)
 @example Get the patient's name value
   name = dcm.value("0010,0010")
 @example Get the Frame of Reference UID from the first item in the Referenced Frame of Reference Sequence
   uid = dcm["3006,0010"][0].value("0020,0052")