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, x is not a janitor. There is at least one x for which x will write a letter if and only if x is a mechanic. For all x, x is a janitor or x will write a letter.
For all x, x will write a letter can be inferred via the universal disjunctive syllogism rule. There is at least one x for which if x writes a letter, then x is a mechanic can be inferred via the existential biconditional elimination rule. There is at least one x for which if x were a mechanic, then x is writing a letter can be inferred via the existential biconditional elimination rule.