You prefer posting on top.
Thanks for this. I do know that questions involving equality and substitutivity for logic programming fall under the term "unification." And, I did spend some time looking at that after reading some of the thread to which you linked.
I am not entirely certain about what you may have read from my post or work, but the syntax of the cyclic terms does reflect my usage.
I do not use the expression "definition" because it has been appropriated by a community who believes their view of logic to have been divinely received at a burning bush.
What I refer to as "symbol introductions" do include statements formulated with a circular syntax when the introduced symbol is a "non-eliminable" relation.
When only a single relation is involved -- for example,
AxAy( x=y <-> Ez( x=z /\ z=y ))
-- what you really have in relation to substitutions in the right hand side is an infinite scheme and a free algebra with infinitely many terms. Unless one fools one's self into making this infinity semantically important, it is harmless -- deflationary.
A slightly more interesting case is when there is a syntactic dependency:
AxAy( x ppart y <-> ( Az( y ppart z -> x ppart z ) /\ Ez( x ppart z /\ ~( y ppart z ) ) ))
AxAy( x in y <-> ( Az( y ppart z -> x in z ) /\ Ez( x in z /\ ~( y ppart z ) ) ))
One can imagine greater complexity, but, it still appears to be harmlesss (or deflationary) Whatever interpretation is given to the relations is unchanged by expanding the right hand sides with substitutions.
The issue here is that relations are a matter of "form" as opposed to "substance." This is why the analysis cannot lead to a classical calculus.
And, in so far as the first statement above is from Tarski's cylindric algebra, departure from a classical calculus is related to algebraization. Appendix C from Blok and Pigozzi is clear about how algebraization necessitates a change to inference rules.
There is a sequent in homotopy type theory,
C; a
----------
C; a=a
My work divides this into two paradigms. Above the line is "objectual." Below the line is "relational." The latter, then, becomes an aspect of "differential ontology."
Related to this is the truth preservation of universals for submodels and the truth preservation of existentials for extensions from first-order model theory.
For the existentials, the symbol introduction for equality is accomplished with Tarski's axiom from above.
For the universals, one has,
AxAy( x=y <-> ( Az~( ~x=z <-> ( x=z /\ z=y ) ) /\ Az~( ~y=z <-> ( y=z /\ x=z ) ) ))
Although I have not checked this with derivations, this ought to be compatible with the "objectual ontology" of first-order inference. This merely ensures that substitutivity is compatible with transitivity for an otherwise unspecified domain of discourse.
Relating equality to a universal in this way admits the possibility of distinguishing between formalism and logicism.
Let a discernibility relation be introduced with
AxAy( x!=y <-> ( y!=y /\ (( y!=y -> y!=x ) /\ ( y!=x -> x!= y )) ))
Because their are no nested quantifiers, the exclusive disjunction,
AxAy~( x=y <-> x!=y )
yields an equality that is interpretable for a first-order formalist ontology. The "self-identity" is derived from the syntax of the discernibility relation.
Let a distinctness relation be introduced with
AxAy( x=/=y <-> ( Az~( ~x=z <-> ( x=z /\ z=/=y ) ) \/ Az~( ~y=z <-> ( y=z /\ x=/=z ) ) ))
Notice that a denied existence will convey existential import. So, the exclusive disjunction,
AxAy~( x=y <-> x=/=y )
yields an equality interpretable as a metaphysical law of identity.
In any case, there are many places in ordinary mathematics where transitivity serves to "glue" structures together. This informal term -- namely, "gluing" -- is found in topology.
I think, however, that it also reflects much of what I read about "unification."
The thread to which you linked also made a comparison with "undefined" situations.
Mitchell Spector's contribution to the October 2015 thread on free logic,
https://cs.nyu.edu/pipermail/fom/2015-October/019285.html
is a simple statement suggesting that the sign of equality can provide the means for sorting through these issues.
Again, though, thanks for the link to the thread. I know very little about Prolog and its methods.