" But "so leaves one possibility less for his successors" does not seem to be an independent clause, for it does not have a subject. The rule is true (except for very short sentences) but it refers to one comma not used otherwise. Here, the comma before 'and' pairs with the comma before 'but' to set off the enclosed unit.
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flowersun2013I thought when "and" joins two independent clauses, a comma must be put before "and." But "so leaves one possibility less for his successors" does not seem to be an independent clause, for it does not have a subject.The rule is true (except for very short sentences) but it refers to one comma not used otherwise. Here, the comma before 'and' pair