Input file

The input file consists of three parts:

1. Rules
2. Initial facts
3. Queries

Example of the input file:

# Rules
B + C => A
D | E => B
B => C

# Initial facts
=D

# Queries
?A

Also, a file can contain comments — a part of the line which starts with #.

Rules

Rules are equations that combine facts with operators.

Facts

Facts represented by uppercase letters.

Operators

Operators can be one of:

• ( and ) — Parenthesis
• ! — NOT (Negation)
• + — AND
• ^ — XOR
• | — OR
• => — Implication

Note: The support of the equivalence operator (<=>) can be implemented as a bonus.

Let's take a look at the truth table of each operator:

! (NOT)

A	!A
false	true
true	false

+ (AND)

A	B	A + B
false	false	false
false	true	false
true	false	false
true	true	true

^ (XOR)

A	B	A ^ B
false	false	false
false	true	true
true	false	true
true	true	false

| (OR)

A	B	A | B
false	false	false
false	true	true
true	false	true
true	true	true

=> (Implication)

A	B	A => B
false	false	true
false	true	true
true	false	false
true	true	true

Each rule should have exactly one implication operator.

An implication operator divides each rule into two parts: a condition (left part) and a conclusion (right part).

The result of implication must be equal to true because the rule must be truthy.

Initial facts

Each input file must have a line that starts with = — initial facts.

After = must be listed facts which will receive the initial value true.

The list of initial facts also can be empty:

=

Queries

Also, the file must contain the “Queries” part — a line that starts with ?.

After ? must be listed facts whose value must be returned (displayed) as a result of expert system calculations.