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: Richard is writing a letter if and only if Elisabeth is drinking water. For all x_10, if x_10 drinks water, then x_10 is excited.
If Richard writes a letter, then Elisabeth is drinking water can be inferred via the biconditional elimination rule. If Elisabeth drinks water, then Richard is writing a letter can be inferred via the biconditional elimination rule.