The following declarations are typically only needed when customizing the schema used by LaTeXML.
RegisterNamespace($prefix,$url) associates the prefix with the given namespace url. This allows you to use $prefix as a namespace prefix when writing Constructor patterns or XPath expressions.
Tag($tag,properties) specifies properties for the given XML $tag. Recognized properties include: autoOpen=>1 indicates that the tag can automatically be opened if needed to create a valid document; autoClose=>1 indicates that the tag can automatically be closed if needed to create a valid document; afterOpen=>$code specifies code to be executed before opening the tag; the code is passed the Document being constructed as well as the Box (or Whatsit) responsible for its creation; afterClose=>code similar to afterOpen, but executed after closing the element.