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: William is a taxi driver if and only if James is a mechanic. James is a mechanic or William is happy. For all x_10, if x_10 were happy, then x_10 is a taxi driver.
If William writes a letter, then James is a mechanic can be inferred via the rewriting rule. If William were a taxi driver, then James is a mechanic can be inferred via the biconditional elimination rule. If James were a mechanic, then William is a taxi driver can be inferred via the biconditional elimination rule.