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_7, x_7 will play squash if and only if x_7 is a mechanic. Michael is a mechanic if and only if David is sad. David is not sad.
If Michael were a mechanic, then David is sad can be inferred via the biconditional elimination rule. If David were sad, then Michael is a mechanic can be inferred via the biconditional elimination rule. Michael is not a mechanic can be inferred via the biconditional elimination rule. For all x_7, if x_7 plays squash, then x_7 is a mechanic can be inferred via the universal biconditional elimination rule. For all x_7, if x_7 were a mechanic, then x_7 is playing squash can be inferred via the universal biconditional elimination rule.