Consider the following premises. There is at least one x_7 for which x_7 will make tea or x_7 will not play squash. For all x_7, x_7 will play squash or x_7 will not make tea. Can we infer the following from them? If we can, name the inference rule being used: There is at least one x_7 for which x_7 will make tea if and only if x_7 will play squash.
 From the fact that there is at least one x_7 for which x_7 will make tea or x_7 will not play squash we can infer that there is at least one x_7 for which if x_7 plays squash, then x_7 is making tea via existential rewriting. From the fact that for all x_7, x_7 will play squash or x_7 will not make tea we can infer that for all x_7, if x_7 makes tea, then x_7 is playing squash via universal rewriting. Finally, from the fact that for all x_7, if x_7 makes tea, then x_7 is playing squash, and that there is at least one x_7 for which if x_7 plays squash, then x_7 is making tea we can infer that there is at least one x_7 for which x_7 will make tea if and only if x_7 will play squash via existential biconditional introduction. Therefore, the answer is yes.