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: If Elisabeth writes a letter, then Robert is sleeping. Elisabeth is writing a letter or Robert is climbing a mountain. For all x_10, if x_10 climbs a mountain, then x_10 is sleeping. For all x_10, if x_10 sleeps, then x_10 is climbing a mountain.
If Robert were sad, then Elisabeth is writing a letter can be inferred via the rewriting rule. For all x_10, x_10 will climb a mountain if and only if x_10 will sleep can be inferred via the universal biconditional introduction rule. For all x_10, x_10 will sleep if and only if x_10 will climb a mountain can be inferred via the universal biconditional introduction rule.