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: James is an office clerk if and only if David is a sailor. For all x_4, x_4 will write a letter if and only if x_4 is a sailor. David is writing a letter or James is an office clerk.
If James were an office clerk, then David is a sailor can be inferred via the biconditional elimination rule. If David were a sailor, then James is an office clerk can be inferred via the biconditional elimination rule. For all x_4, if x_4 writes a letter, then x_4 is a sailor can be inferred via the universal biconditional elimination rule. For all x_4, if x_4 were a sailor, then x_4 is writing a letter can be inferred via the universal biconditional elimination rule.