System ST β-reduction and completeness

We prove that system ST (introduced in a previous work) enjoys subject reduction and is complete for realizability semantics. As far as the author knows, this is the only type system enjoying the second property. System ST is a very expressive type system, whose principle is to use two kinds of formulae: types (formulae with algorithmic content) and propositions (formulae without algorithmic content). The fact that subtyping is used to build propositions and that propositions can be used in types through a special implication gives its great expressive power to the system: all the operators you can imagine are definable (union, intersection, singleton, ...).