def is_false(self, e, extra_constraints=(), solver=None, model_callback=None): 
<EOL>if not isinstance(e, Base):<EOL><INDENT>return self._is_false(self.convert(e), extra_constraints=extra_constraints, solver=solver, model_callback=model_callback)<EOL><DEDENT>try:<EOL><INDENT>return self._false_cache[e.cache_key]<EOL><DEDENT>except KeyError:<EOL><INDENT>f = self._is_false(self.convert(e), extra_constraints=extra_constraints, solver=solver, model_callback=model_callback)<EOL>self._false_cache[e.cache_key] = f<EOL>if f is True:<EOL><INDENT>self._true_cache[e.cache_key] = False<EOL><DEDENT>return f<EOL><DEDENT>
Should return True if e can be easily found to be False.

:param e:                   The AST
:param extra_constraints:   Extra constraints (as ASTs) to add to the solver for this solve.
:param solver:              A solver, for backends that require it
:param model_callback:      a function that will be executed with recovered models (if any)
:return:                   A boolean.