A qualitative probability satisfies some axioms:
* QP1
* QP2
* QP3
Lemma "=>" a qualitative probability =>
* Forall B in A, O"=>"B
* forall B,C in A, B Subset C => B "<" C
* forall B,C,D,E in A, B intersect C = empty set, D intersect E= empty set, B"=>"D, C"=>"E => A union B "=>" C union D