Atom (expression) design

[0xGeorgii]

Some ideas from Handbook of Practical Logic and Automated Reasoning

[0xGeorgii]

It seems from the definition

    type Formula =
        | Var of string
        | Const of bool
        | Disj of Formula * Formula
        | Conj of Formula * Formula
        | Neg of Formula
        | Bic of Formula * Formula
        | Impl of Formula * Formula

Atom is a Var (atomic formula) = string

By a definition ref: Symbolic Logic and Mechanical Theorem Proving (Computer Science and Applied Mathematics) 1st Edition, by Chin-Liang Chang, Richard Char-Tung Lee, p. 7