A notation tag is an element <tag ..> which can have the following attributes:

• type - the type of the tag, notation or variant can be specified. If it's omitted, then the tag is valid in all cases.
• weight - a positive number denoting the relevance of the tag. The greater the number the more relevant is the tag.
• xref - white-space separated URIs to specific target elements for which this tag is valid. For the notations, this would include references to renderings. If @xref is not specified, it's valid for any element. TODO: fix it, currently it only works for one URI?
• context - key-value pairs of contextual information, in the same format as for the ic attribute. Note: maybe it should be renamed to ic for consistency?
• object - white-space separated URIs to specific [math] objects for which this tag is valid. For the notations, this would reference the objects being rendered.
• owner - document author.

Original ticket #182