The first argument to a cref command is the text
that the user sees for the link.
This is also the default value for the cross reference tag of the
section that we are linking to.
Each section starts with a begin command
that contains its cross reference tag as an argument.
The cross reference tag for this section is cref_example_1.
The cross reference tags for the children of this section are
Note that the sections corresponding to
are automatically generated.