Table of Contents - 21.36.6.11. _Begriffsschrift_ Chapter III Single-valued procedures
means the relationship content of procedure is
single-valued. The double converse allows to simply apply this syntax in
place of Frege's even though the original never explicitly limited discussion
of propositional statements which vary on two variables to relations.
dffrege115 through frege133 develop this and how functions relate
to transitive and transitive-reflexive closures.