def _eval(self, expr, n, extra_constraints=(), solver=None, model_callback=None): 
raise BackendError("<STR_LIT>")<EOL>
This function returns up to `n` possible solutions for expression `expr`.

:param expr:                An expression (backend object) to evaluate.
:param n:                   A number of results to return.
:param extra_constraints:   Extra constraints (as ASTs) to add to the solver for this solve.
:param solver:              A solver object, native to the backend, to assist in the evaluation (for example, a
                            z3.Solver).
:param model_callback:      a function that will be executed with recovered models (if any)
:return:                    A sequence of up to n results (backend objects).