e="font-family: arial; font-weight: bold;">Language</span><small
 style="font-family: arial;"><sup>&#8869;</sup></small><span
 style="font-family: arial; font-weight: bold;">
</span>is the subcontext whose morphisms are special &#8211; they have
bijective
variable functions.<br>
</p>
<p style="text-align: justify;">Any type language is extended to a type
language of <span style="font-style: italic;">expressions</span>,
which has the same sets for variables and
entity types and the same reference function, b