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This mod is an addon for the GameStages API. Dimension Stages allows access to dimensions to be gated behind custom progression systems that are set in place by the modpack creator. If a player does not have access to a stage, they will not be able to enter that dimension. Their pets, and projectiles, mounts and dropped items will also be prevented from entering a restricted dimension. You can find more info about Dimension Stages here

//mods.DimensionStages.addDimensionStage(String stage, int dimensionId);
mods.DimensionStages.addDimensionStage("nether", -1);