@Section
    @Title { Cross references }
@Begin
@PP
Cross references, such as `see page 57' and `see Figure 5,' are a useful
but highly error-prone feature of documents.  Scribe [7]
introduced a method of keeping them up to date automatically as the
document changes:  the user gives each referenced entity a tag, and
operators are provided that return the page or sequence number of the
entity with a given tag.
@PP
A cross reference takes an object (such as a page number) from one point
in the document and copies it to another, and this generalization
suggests other applications.  For example, a running header is copied
from the title of a nearby chapter, and a reference is copied from a
bibliographic database.  Making the unity of these applications manifest
is an interesting language design problem.
@BeginSubSections