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_15, x_15 is a carpenter if and only if x_15 will write a letter. For all x_15, if x_15 climbs a mountain, then x_15 is writing a letter. For all x_15, x_15 is an electrician if and only if x_15 will write a letter. For all x_15, if x_15 were a carpenter, then x_15 is sad. For all x_15, x_15 will climb a mountain or x_15 is an electrician.
For all x_15, if x_15 were a carpenter, then x_15 is writing a letter can be inferred via the universal biconditional elimination rule. For all x_15, if x_15 were an electrician, then x_15 is writing a letter can be inferred via the universal biconditional elimination rule. For all x_15, if x_15 writes a letter, then x_15 is a carpenter can be inferred via the universal biconditional elimination rule. For all x_15, if x_15 writes a letter, then x_15 is an electrician can be inferred via the universal biconditional elimination rule.