Use Atomese Formulas, and not scheme, for math

[linas]

Friendly reminder, because I think it has fallen off the TODO-list. The new atomese formula support is likely to be a lot faster than using (GroundedPredicateNode "scm:foo-formula") See https://github.com/opencog/atomspace/blob/master/examples/atomspace/formulas.scm for a working example; I copy part of it below to give a flavor of it:

; Create a SimpleTruthValue with a non-trivial formula:
; It will be the TV := (1-sA*sB, cA*cB) where sA and sB are strenghts
; and cA, cB are confidence values. The PredicateFormulaLink assembles
; two floating-point values, and create a SimpleTruthValue out of them.
;
(cog-evaluate!
   (PredicateFormula
      (Minus
         (Number 1)
         (Times (StrengthOf (Concept "A")) (StrengthOf (Concept "B"))))
      (Times (ConfidenceOf (Concept "A")) (ConfidenceOf (Concept "B")))))

; The values do not need to be formulas; they can be hard-coded numbers.
(cog-evaluate!
   (PredicateFormula (Number 0.7) (Number 0.314)))

; The below computes a truth value, and attaches it to the
; EvaluationLink. Let's demo this in three parts: first define it,
; then evaluate it, then look at it.
;
(define my-ev-link
   (Evaluation
      (PredicateFormula (Number 0.7) (Number 0.314))
      (List
         (Concept "A")
         (Concept "B"))))

; Evaluate ...
(cog-evaluate! my-ev-link)

; Print.
(display my-ev-link)

; More typically, one wishes to have a formula in the abstract,
; with variables in it, so that one can apply it in any one of
; a number of different situations. In the below, the variables
; are automatically reduced with the Atoms in the ListLink, and
; then the formula is evaluated to obtain a TruthValue.
(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"))))

and so on. The example gives even more details that make it even more PLN-useful.

[linas]

See also opencog/atomspace#2532 and see also examples/atomspace/flows.scm and examples/atomspace/flow-formulas.scm and also the blog entry https://blog.opencog.org/2020/04/08/value-flows/