func RegisterURN(urn string) {
	urnsCache = urnsCache.insert(urnsCache, urn, 0)
}
RegisterURN saves the given urn into the lookup structure, so it could be referenced later. This is necessary because
the URN prefix defined in SCIM breaks ordinary path syntax by the use of dot (.). Normally, dot is used to separate
path segments (i.e. name.familyName). However, dot is also contained in URN prefix such as
urn:ietf:params:scim:schemas:core:2.0:User
to indicate version 2.0. Hence, the compiler needs to recognize these URN prefixes in advance to properly parse them
as a path segment instead of delimiting by dot.
package expr