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Propositional logic theorem prover
*CMD CanProve  try to prove statement
*STD
*CALL
CanProve(proposition)
*PARMS
{proposition}  an expression with logical operations
*DESC
Yacas has a small builtin propositional logic theorem prover.
It can be invoked with a call to {CanProve}.
An example of a proposition is: "if a implies b and b implies c then
a implies c". Yacas supports the following logical operations:
{Not} : negation, read as "not"
{And} : conjunction, read as "and"
{Or} : disjunction, read as "or"
{=>} : implication, read as "implies"
The abovementioned proposition would be represented by the following expression,
( (a=>b) And (b=>c) ) => (a=>c)
Yacas can prove that is correct by applying {CanProve}
to it:
In> CanProve(( (a=>b) And (b=>c) ) => (a=>c))
Out> True;
It does this in the following way: in order to prove a proposition $p$, it
suffices to prove that $Not p$ is false. It continues to simplify $Not p$
using the rules:
Not ( Not x) > x
(eliminate double negation),
x=>y > Not x Or y
(eliminate implication),
Not (x And y) > Not x Or Not y
(De Morgan's law),
Not (x Or y) > Not x And Not y
(De Morgan's law),
(x And y) Or z > (x Or z) And (y Or z)
(distribution),
x Or (y And z) > (x Or y) And (x Or z)
(distribution),
and the obvious other rules, such as,
True Or x > True
etc.
The above rules will translate a proposition into a form
(p1 Or p2 Or ...) And (q1 Or q2
Or ...) And ...
If any of the clauses is false, the entire expression will be false.
In the next step, clauses are scanned for situations of the form:
(p Or Y) And ( Not p Or Z) > (Y Or Z)
If this combination {(Y Or Z)} is empty, it is false, and
thus the entire proposition is false.
As a last step, the algorithm negates the result again. This has the
added advantage of simplifying the expression further.
*E.G.
In> CanProve(a Or Not a)
Out> True;
In> CanProve(True Or a)
Out> True;
In> CanProve(False Or a)
Out> a;
In> CanProve(a And Not a)
Out> False;
In> CanProve(a Or b Or (a And b))
Out> a Or b;
*SEE True, False, And, Or, Not
