Consider the following premises. For all x_12, x_12 will not climb a mountain. For all x_12, x_12 will play squash if and only if x_12 is a sailor. There is at least one x_12 for which x_12 will play squash if and only if x_12 will climb a mountain. Can we infer the following from them? For all x_12, x_12 is a sailor.
 From the fact that for all x_12, x_12 will play squash if and only if x_12 is a sailor we can infer that for all x_12, if x_12 were a sailor, then x_12 is playing squash. From the fact that there is at least one x_12 for which x_12 will play squash if and only if x_12 will climb a mountain, and that for all x_12, x_12 will not climb a mountain we can infer that there is at least one x_12 for which x_12 will not play squash. Finally, from the fact that for all x_12, if x_12 were a sailor, then x_12 is playing squash, and that there is at least one x_12 for which x_12 will not play squash we can infer that there is at least one x_12 for which x_12 is not a sailor, which contradicts that For all x_12, x_12 is a sailor. Therefore, the answer is no.