def _solution(self, expr, v, extra_constraints=(), solver=None, model_callback=None): 
raise BackendError("<STR_LIT>")<EOL>
Return True if v is a solution of expr with the extra constraints, False otherwise.

:param expr: expression (backend object) to evaluate
:param v: the proposed solution (backend object)
:param solver: a solver object, native to the backend, to assist in
               the evaluation (for example, a z3.Solver)
:param extra_constraints: extra constraints (as ASTs) to add to the solver for this solve
:return: True if v is a solution of expr, False otherwise