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: Mary is not a sailor. Mary is writing a letter if and only if it is cloudy. For all x_3, x_3 is a sailor if and only if x_3 will write a letter.
If Mary writes a letter, then it is cloudy can be inferred via the biconditional elimination rule. If it is cloudy, then Mary is writing a letter can be inferred via the biconditional elimination rule. For all x_3, if x_3 were a sailor, then x_3 is writing a letter can be inferred via the universal biconditional elimination rule. For all x_3, if x_3 writes a letter, then x_3 is a sailor can be inferred via the universal biconditional elimination rule.