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