ut has logical
expressions (formulas) as its relation types. The set of expressions,
which is defined recursively, is the disjoint union of atomic
expressions, negations, conjunctions, disjunctions, implications,
existential and universal quantifications, and substitutions. There is
a special <span style="font-style: italic;">embedding </span>type
language morphism from any type language to
its expression type language. Of course, we can form expressions of
expressions. There is also a special <span style="