What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? For all x_13, if x_13 writes a letter, then x_13 is fast. For all x_13, if x_13 were fast, then x_13 is writing a letter. David is not running or David is working. David is running or Robert is writing a letter. For all x_13, x_13 is not fast.
Robert is writing a letter or David is working. If Robert cooks, then David is running. For all x_13, x_13 will not write a letter. For all x_13, x_13 will write a letter if and only if x_13 is fast. For all x_13, x_13 is fast if and only if x_13 will write a letter.