Write for a DTD definition that best fulfills the following conditions:

Information for a multiple number of products coded within in one XML document. Character data can be coded for product names $($element name: product$).$ The product number must have a single, unique name $($attribute name: partsno$),$ and is verified as singular and unique by the XML parser. Characters such as # and @ must not be used in price information $($attribute name: price$)$ The default value for price $($attribute name: price$)$ is "open". The "product" element must have a defined "partsno" attribute.