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_15, x_15 is not excited. For all x_15, x_15 is sad if and only if x_15 will read a book. For all x_15, x_15 is excited or x_15 is not sad. Robert is reading a book if and only if it is cloudy.
If Robert reads a book, then it is cloudy can be inferred via the biconditional elimination rule. If it is cloudy, then Robert is reading a book can be inferred via the biconditional elimination rule. For all x_15, if x_15 were sad, then x_15 is excited can be inferred via the universal rewriting rule. For all x_15, if x_15 were sad, then x_15 is reading a book can be inferred via the universal biconditional elimination rule. For all x_15, if x_15 reads a book, then x_15 is sad can be inferred via the universal biconditional elimination rule.