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Common Logic (CL) and IKL — Pat Hayes and Chris Menzel's extension of it — are read by a CLIF reader implemented in Lean 4. This page is the tutorial: it works through the IKL GUIDE itself, section by section, quoting the GUIDE's own prose and parsing the GUIDE's own examples with the same reader, live.

What the reader will not read#

Before the tour starts, one boundary. ISO/IEC 24707 wraps a body of CL text in cl:text/cl:comment/cl:module/cl:imports phrase forms — the namespace-qualified spelling of the unprefixed text/comment forms the GUIDE's own Appendix A uses to wrap its structural axioms ("It is written entirely as commented IKL text"). This reader parses single sentences and flat sentence sequences (CL/Syntax.lean's covered fragment); it does not parse that enclosing structure, so asking it to read one exercises the boundary directly.

moduleForm = '(cl:text "demo" (P a))'

moduleAttempt = {
  try { return await fn.l4Call("clParse", [moduleForm]); }
  catch (e) { return { ok: false, message: e.message }; }
}

The error names the reason and the tracking issue: 'cl:text' phrases are not covered by this reader (issue 580). The engine stays alive afterward — every op call below runs against the same module.

Predication and names (GUIDE "IKL Overview")#

CLIF — Common Logic Interchange Format — is, in the GUIDE's own words, written "in 'Lisp style', with the relation or function name after the opening parenthesis, and with names separated by whitespace rather than commas." Ground facts are the simplest sentences it has: the GUIDE's "Description logics translate into IKL relational operators" section gives two, back to back — "Ground facts (often called 'A-box' sentences in the DL literature) such as membership in a class, or a property having a value, are represented as a simple atomic sentences such as (isHuman "Osama bin Laden")(childOf "Osama bin Laden" "Hamida al-Attas")."

groundFacts = '(isHuman "Osama bin Laden")(childOf "Osama bin Laden" "Hamida al-Attas")'

groundParse = fn.l4Call("clParse", [groundFacts])

sentences reads 2 — one text, two sentences — and pureCL is true: nothing here is IKL-specific, it is ISO/IEC 24707 CL proper. The double quotes around "Osama bin Laden" are CLIF's enclosed name form — a normal name spelled with whitespace, not the single-quoted quoted string that the GUIDE distinguishes carefully in "Special IKL name forms": an enclosed name "function[s] logically just like any other name," while a quoted string "denotes a particular sequence of ... Unicode characters, and cannot mean anything else." normalized re-encloses both names on the way back out, unchanged in meaning.

Quantifier forms#

The GUIDE's "Forms of quantifiers" section adds a restriction to a quantifier binding by writing (name term) inside the binding list, "an abbreviation" the GUIDE spells out directly: (forall ((x isHuman))(exists ((y charseq))(= y (nameOf x)) )).

restrictedQuant = "(forall ((x isHuman))(exists ((y charseq))(= y (nameOf x))))"

restrictedParse = fn.l4Call("clParse", [restrictedQuant])

pureCL is true — restricted binders are CLIF sugar, not an IKL extension — and normalized shows the reader's own spacing of the same restricted forms, (forall ((x isHuman)) (exists ((y charseq)) (= y (nameOf x)))).

Proposition names and (that S)#

The GUIDE's "Proposition names" section introduces the construct that makes IKL more than Common Logic: "a syntactic form which makes a sentence into the name of the corresponding proposition, by enclosing it inside parentheses, preceded by the special IKL reserved word that." To assert a proposition rather than merely name it, the GUIDE adds a second pair of parentheses around the name, "cancelling" the reification: ((that (isHuman "Brant Cheikes"))) — its own example — "is such an atomic sentence, which means exactly the same as the inner sentence."

cheikes = '((that (isHuman "Brant Cheikes")))'

cheikesParse = fn.l4Call("clParse", [cheikes])

pureCL is false: the cancelling-parentheses form is IKL, not plain CL, even though it asserts nothing an ordinary sentence could not.

Quantifying-in#

"Quantifying in" is the GUIDE's name for a proposition ABOUT a particular, unnamed individual — an outer quantifier binding a variable that reappears free inside a (that S) term. Its own example, from "Proposition names": (exists (x) (Believes "Lois Lane" (that (= x Superman)) )).

loisLane = '(exists ((x isHuman)) (Believes "Lois Lane" (that (= x Superman)) ))'

loisParse = fn.l4Call("clParse", [loisLane])

It parses (pureCL is false, as any that-bearing sentence is).

What identity does not include#

Appendix B, "Identity Conditions for Propositions," fixes bound-name renaming as the floor on propositional identity, stated directly: "a change of bound names in a sentence does not change the proposition expressed by the sentence, so that for example (that (exists (x)(loves Jim x))) equals (that (exists (y)(loves Jim y)))." That much is a naming-independent fact about the two sentences, provable from the CLIF grammar alone — it needs no RDF projection to state or check.

Appendix B then goes further, adding a separate, DEFINED relation for a wider set of equivalences: =p holds of commuted conjunction — (=p (that (and PHI RHO)) (that (and RHO PHI)) ) — as one of several axioms the GUIDE lists explicitly, allowing "propositional identity to be defined by axioms," with =p "the smallest relation satisfying all these conditions." Bound-name renaming and =p-commutativity are different claims of different strength: the first follows from the grammar of quantification, the second is an axiom asserted on top of it. Recovering that second, wider relation from an RDF encoding is tracked at issue 589 and is not demonstrated on this page.

Closing#

Every CLIF text on this page is quoted or directly adapted from the IKL GUIDE (Hayes and Menzel): "Description logics translate into IKL relational operators" for ground predication; "Forms of quantifiers" for the restricted-binder abbreviation; "Proposition names" for the cancelling-parentheses assertion and quantifying-in; Appendix B for the bound-name-renaming floor and the =p commutativity axiom. One place said plainly where the reader's covered fragment (CL/ Syntax.lean) stops short of the GUIDE's own text: the cl:text phrase-structure boundary. The wider CL/IKL port is tracked at issue 580; proposition individuation — bound-name renaming versus the GUIDE's stronger =p — at issue 589.