What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? Name the inference rule being used: There is at least one x_1 for which x_1 is not a taxi driver. For all x_1, x_1 will drink water if and only if x_1 is a taxi driver. For all x_1, x_1 will drink water if and only if x_1 is sad.
For all x_1, if x_1 drinks water, then x_1 is a taxi driver can be inferred via the universal biconditional elimination rule. For all x_1, if x_1 drinks water, then x_1 is sad can be inferred via the universal biconditional elimination rule. For all x_1, if x_1 were a taxi driver, then x_1 is drinking water can be inferred via the universal biconditional elimination rule. For all x_1, if x_1 were sad, then x_1 is drinking water can be inferred via the universal biconditional elimination rule. There is at least one x_1 for which x_1 will not drink water can be inferred via the existential biconditional elimination rule.