An expression can either be a propositional constant, predicate, or compound expression
Propositional constants represent a true or false statement. In the editor they are represented as a single fully connected alphanumeric string not enclosed by any parenthesis. Some examples:
x
phi
psi
Logical Negation of an expression A
(not A)
Logical And between two expressions A, B
(and A B)
Logical Or between two expressions A, B
(or A B)
Material Implication between two expressions A, B
(if A B)
Logical biconditional between two expressions A, B
(iff A B)
Some example compound expressions:
phi and psi
(and phi psi)
if phi then A or B
(if phi (or A B))