|Description: In the paper "On
Variable Functors of Propositional Arguments",
Lukasiewicz introduced a system that can handle variable connectives.
This was done by introducing a variable, marked with a lowercase delta,
which takes a wff as input. In the system, "delta " means that
"something is true of ." "delta " can be substituted with
, , , etc.
Later on, Meredith discovered a single axiom, in the form of delta
delta delta . This represents the shortest theorem
in the extended propositional calculus that cannot be derived as an
instance of a theorem in propositional calculus.
A symmetry with . (Contributed by Anthony Hart,