T</span><small><sub>0</sub></small>)
on the left side of the theory context. The context of theories is
indexed by the context of languages through the base passage <span
 style="font-family: arial; font-style: italic;">base</span><span
 style="font-family: arial; font-style: italic; font-weight: bold;">
</span>: <span style="font-family: arial; font-weight: bold;">Theory</span>
&#8594; <span style="font-family: arial; font-weight: bold;">Language</span>,
which maps a theory to its underlying type
language.