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_13, x_13 is a sailor or x_13 is a nurse. For all x_13, if x_13 were a nurse, then x_13 is writing a letter. For all x_13, x_13 is a sailor if and only if x_13 will write a letter. For all x_5, if x_5 writes a letter, then x_5 is a mechanic.
For all x_13, if x_13 were a sailor, then x_13 is writing a letter can be inferred via the universal biconditional elimination rule. For all x_13, if x_13 writes a letter, then x_13 is a sailor can be inferred via the universal biconditional elimination rule.