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: Sarah is a musician. David is happy if and only if David is writing a letter. For all y_3, y_3 will not write a letter.
If David were happy, then David is writing a letter can be inferred via the biconditional elimination rule. If David writes a letter, then David is happy can be inferred via the biconditional elimination rule.