the ususal 'forall' operator 'all' (be A a formula then
'all x.A' is true if and only if for all elements d of the
universe A{x<-d} (x substitued with d) is true
the ususal 'exists' operator 'ex' (be A a formula then
'ex x.A' is true if and only if there is at least one elements
d of the universe such that A{x<-d} (x substitued with d) is true