This module implements a mechanism of labels and references.
Labels are defined via variables, which are expanded with the
<ref> tag. A common problem is when labels are referenced
before they are defined. For this reason, labels are stored in an
auxiliary file. When the module is loaded, this file is read and
labels are defined. Then input is processed and at end of input, labels
are compared with their previous value. If some labels changed, then
all labels are written to the auxiliary file and a message is printed
to warn that labels are wrong.
