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The seventh son of a seventh son is destined to be a leader of humanity. Since Elon isn't a seventh son himself, he will have to start that line with X Æ A-X, ensuring that X has seven sons himself. Since female siblings ruin the line, selection will need to be used to ensure that X has no daughters until after his seventh son is born.
>The seventh son of a seventh son is a concept from folklore regarding special powers given to, or held by, such a son. To qualify as "the seventh son of a seventh son" one must be the seventh male child born in an unbroken line with no female siblings born between, and to a father who himself is the seventh male child born in an unbroken line with no female siblings born between. The number seven has a long history of mystical and biblical significance, such as seven virtues, seven deadly sins, Seven Sleepers and Seven Heavens.