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: There is at least one x_10 for which x_10 will write a letter. For all x_10, x_10 will play squash or x_10 is sad. For all x_10, if x_10 were sad, then x_10 is not writing a letter. For all x_10, x_10 will not play squash or x_10 will play tennis.
There is at least one x_10 for which x_10 is not sad can be inferred via the existential modus tollens rule. For all x_10, x_10 is sad or x_10 will play tennis can be inferred via the universal resolution rule.