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, if x writes a letter, then x is drinking water. For all x, if x drinks water, then x is writing a letter. Charles is writing a letter.
There is at least one x for which x will write a letter can be inferred via the existential generalization rule. For all x, x will write a letter if and only if x will drink water can be inferred via the universal biconditional introduction rule. For all x, x will drink water if and only if x will write a letter can be inferred via the universal biconditional introduction rule.