A URL can be constructed from a static part, and one or more variables, here called levels. Each level can be a defined list of keys (e.g., letters), a range (numeric from-to, plus step size), or follow (get URLs listed on a page and follow them). The last level with be run through, before the level above it (next lower level) ticks ahead, and the higher level resets.
So, if the first level is keys A-Z, and the second is range 1-100 (step size 1), URLs will use A/1, A/2,... A/100, B/1, B/2,... Z/100.

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At least one level is required