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_13, x_13 is sad if and only if x_13 will make tea. For all x_13, x_13 will make tea if and only if x_13 will play squash. William is sad or James is excited. For all x_13, x_13 will not play squash.
For all x_13, if x_13 were sad, then x_13 is making tea can be inferred via the universal biconditional elimination rule. For all x_13, if x_13 makes tea, then x_13 is playing squash can be inferred via the universal biconditional elimination rule. For all x_13, if x_13 makes tea, then x_13 is sad can be inferred via the universal biconditional elimination rule. For all x_13, if x_13 plays squash, then x_13 is making tea can be inferred via the universal biconditional elimination rule. For all x_13, x_13 will not make tea can be inferred via the universal biconditional elimination rule.