Fix Self-Referential Assignments

[rcosta358]

This PR fixes #176, where a simple assignment x = x + 1 would cause a false negative in the verification.

Root Cause

In the joinPredicates method, creating a new VCImplication with Variable.getRefinement() added the constraint x == lastInstance to connect the variable with its current value. However, when the assignment is self-referential (e.g., x = x + 1), it introduced a contradictory cycle in the SMT premises. Then, Z3 couldn't find a counterexample, therefore concluding the verification was correct.

Solution

Added two helper methods to detect and prevent these cycles:

• hasDependencyCycle: checks if the value of a variable references itself transitively
• hasVariable: checks if a variable is present in a given expression

Then, when a cycle is detected, we use getMainRefinement() instead of getRefinement().