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  "body": "\n## Summary\n\nA series-parallel partial order is a finite poset built from single points by two operations.\nThe series operation puts one poset entirely below another, as in the [[ordinal sum|6AJ7QQCTQQ]].\nThe parallel operation places two posets side by side, as in the [[disjoint union|TZWMEM7GKA]].\nMany problems that are hard for general posets, such as counting linear extensions, become easy for this class.\n\n## Explanations\n\n### L1 human\n\nThink of a project where some groups of tasks run one after another, and other groups run at the same time.\nIf the whole plan can be described this way, by nesting \"then\" and \"at the same time\", its order is series-parallel.\nFor example, \"do $a$, then do $b$ and $c$ at the same time, then do $d$\" gives the poset $a \u003c b, c \u003c d$ with $b$ and $c$ incomparable.\n",
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