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: For all x_10, x_10 will write a letter if and only if x_10 is sad. For all x_10, if x_10 were a police officer, then x_10 is sad. There is at least one x_10 for which if x_10 were sad, then x_10 is a police officer. For all x_10, x_10 will not write a letter.
There is at least one x_10 for which x_10 is sad if and only if x_10 is a police officer can be inferred via the existential biconditional introduction rule. There is at least one x_10 for which x_10 is a police officer if and only if x_10 is sad can be inferred via the existential biconditional introduction rule. For all x_10, if x_10 writes a letter, then x_10 is sad can be inferred via the universal biconditional elimination rule. For all x_10, if x_10 were sad, then x_10 is writing a letter can be inferred via the universal biconditional elimination rule. For all x_10, x_10 is not sad can be inferred via the universal biconditional elimination rule.