The PredicateFormulaLink is an evaluatable Link that can be used to specify arithmetic formulas that compute TruthValues. That is, they allow truth value formulas to be symbolically represented in the AtomSpace, and also be used for performing the actual computation that they represent.

## Example

The following illustrates the representation and computation of a "typical" truth-value formula found in PLN. Start by defining two atoms, with non-trivial TruthValues on them:

```(use-modules (opencog) (opencog exec))

(Concept "A" (SimpleTruthValue 0.8 1.0))
(Concept "B" (SimpleTruthValue 0.6 0.9))
```

Compute a new TruthValue, given by the formula TV := (1-sA*sB, cA*cB):

```(cog-evaluate!
(PredicateFormula
(Minus
(Number 1)
(Times (StrengthOf (Concept "A")) (StrengthOf (Concept "B"))))
(Times (ConfidenceOf (Concept "A")) (ConfidenceOf (Concept "B")))))
```

Formulas can be attached to EvaluationLinks in the usual fashion:

```(cog-evaluate!
(Evaluation
; Compute TV = (1-sA*sB, cA*cB)
(PredicateFormula
(Minus
(Number 1)
(Times
(StrengthOf (Variable "\$X"))
(StrengthOf (Variable "\$Y"))))
(Times
(ConfidenceOf (Variable "\$X"))
(ConfidenceOf (Variable "\$Y"))))
(List
(Concept "A")
(Concept "B"))))
```

The above is visually awkward; this can be overcome with the DefineLink. For example, one imagines that the formula might be computing the degree of reddish color of some objects; one would then define

```(DefineLink
(DefinedPredicate "has a reddish color")
(PredicateFormula
(Minus
(Number 1)
(Times
(StrengthOf (Variable "\$X"))
(StrengthOf (Variable "\$Y"))))
(Times
(ConfidenceOf (Variable "\$X"))
(ConfidenceOf (Variable "\$Y")))))
```

which is then used to give the EvaluationLink a very natural-looking form:

```(cog-evaluate!
(Evaluation
(DefinedPredicate "has a reddish color")
(List
(Concept "A")
(Concept "B"))))
```

Whenever the TruthValue on `(Concept "A")` or `(Concept "B")` changes, the above can be re-run to obtain a new TruthValue on the EvaluationLink.

A more detailed tutorial can be found in the /examples/atomspace/formulas.scm file.