10. An apparatus comprising: a processing unit; a proof obtainer operative to obtain a proof of a property with respect to a bounded model having a bounded number of cycles, wherein the bounded model comprising an initial axiom and a transition relation axiom, wherein the proof of the property is a Directed Acyclic Graph (DAG), wherein each non-leaf node of the DAG is deducible from its child nodes, wherein a root of the DAG is the property, and wherein leaves of the DAG are associated with an axiom of the bounded model; a candidate set selector operative to select a set of candidate invariants comprising at least one intermediate node of the DAG; and an invariant determinator operative to determine, without using the proof and using a Boolean satisfiability problem solver, a subset of the set of candidates, wherein the subset comprises invariants which are held in an unbounded model during each cycle after the bound, wherein the unbounded model is an unbounded version of the bounded model.
10. An apparatus comprising: a processing unit; a proof obtainer operative to obtain a proof of a property with respect to a bounded model having a bounded number of cycles, wherein the bounded model comprising an initial axiom and a transition relation axiom, wherein the proof of the property is a Directed Acyclic Graph (DAG), wherein each non-leaf node of the DAG is deducible from its child nodes, wherein a root of the DAG is the property, and wherein leaves of the DAG are associated with an axiom of the bounded model; a candidate set selector operative to select a set of candidate invariants comprising at least one intermediate node of the DAG; and an invariant determinator operative to determine, without using the proof and using a Boolean satisfiability problem solver, a subset of the set of candidates, wherein the subset comprises invariants which are held in an unbounded model during each cycle after the bound, wherein the unbounded model is an unbounded version of the bounded model. 16. The apparatus of claim 10 , wherein said candidate set selector is operative to select substantially all nodes as invariant candidates.