Useful unused data and routines

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+This page is for pointing out unused data or routines that were left over in the pokecrystal ROM.
+
+
+## Contents
+
+- [NPC sprites](#npc-sprites)
+- [Pink overworld sprite color](#pink-overworld-sprite-color)
+- [Map environment for outdoor dungeons](#map-environment-for-outdoor-dungeons)
+- [Tile collision behavior](#tile-collision-behavior)
+- [Held item effects to prevent status conditions](#held-item-effects-to-prevent-status-conditions)
+- [Experience growth rates](#experience-growth-rates)
+- [NPCs that trade for a male Pokémon](#npcs-that-trade-for-a-male-pokémon)
+
+
+## NPC sprites
+
+From [constants/sprite_constants.asm](../blob/master/constants/sprite_constants.asm):
+
+- `SPRITE_UNUSED_GUY`: An old balding man with a moustache. Does not have walking frame graphics. Maybe the Safari Zone Warden?
+- `SPRITE_OLD_LINK_RECEPTIONIST`: The Cable Club receptionist from RBY.
+
+Use them in [maps/\*.asm](../tree/master/maps/).
+
+
+## Pink overworld sprite color
+
+From [constants/sprite_data_constants.asm](../blob/master/constants/sprite_data_constants.asm):
+
+- `PAL_NPC_PINK`: A counterpart to the other `PAL_NPC_*` constants. Pink looks very similar to red. (IMO purple would be more useful.)
+
+
+## Map environment for outdoor dungeons
+
+From [constants/map_data_constants.asm](../blob/master/constants/map_data_constants.asm):
+
+- `ENVIRONMENT_5`: An environment that seems neither outdoors nor indoors. You can't use Fly, Teleport, Dig, Escape Rope, or the Bicycle; and its colors change with the time of day, but unlike `TOWN` and `ROUTE` maps, they don't use the special outdoor water colors.
+
+Use it in [data/maps/maps.asm](../blob/master/data/maps/maps.asm).
+
+
+## Tile collision behavior
+
+From [constants/collision_constants.asm](../blob/master/constants/collision_constants.asm):
+
+- `COLL_CURRENT_*`: Force the player to Surf `RIGHT`, `LEFT`, `UP`, or `DOWN`. Similar to the current tiles in RSE west of Pacifidlog Town.
+- `COLL_WALK_*` and `COLL_BRAKE`: Force the player to walk `RIGHT`, `LEFT`, `UP`, or `DOWN`, or to stop forced walking. Similar to the spinner tiles in RBY's Rocket Hideout, but without the spinning.
+- `COLL_HOP_UP`, `COLL_HOP_UP_RIGHT`, and `COLL_HOP_UP_LEFT`: Counterparts to the other `COLL_HOP_*` constants. Useful for upward-facing ledges.
+- `COLL_DOWN_WALL`: A counterpart to `COLL_RIGHT_WALL`, `COLL_LEFT_WALL`, and `COLL_UP_WALL`. Blocks the bottom edge but not the other three. Useful for placing above cave entrances in case the player can walk on top of cliffs with caves.
+
+Use them in [data/tilesets/\*_collision.asm](../tree/master/data/tilesets/).
+
+
+## Held item effects to prevent status conditions
+
+From [constants/item_data_constants.asm](../blob/master/constants/item_data_constants.asm):
+
+- `HELD_PREVENT_*`: For held items that prevent status conditions: `POISON`, `BURN`, `FREEZE`, `SLEEP`, `PARALYZE`, or `CONFUSE`.
+
+Use it in [data/items/attributes.asm](../blob/master/data/items/attributes.asm).
+
+
+## Experience growth rates
+
+From [constants/pokemon_data_constants.asm](../blob/master/constants/pokemon_data_constants.asm):
+
+- `GROWTH_SLIGHTLY_FAST`: Needs 849,970 to reach level 100.
+- `GROWTH_SLIGHTLY_SLOW`: Needs 949,930 to reach level 100.
+
+Use them in [data/pokemon/base_stats/\*.asm](../tree/master/data/pokemon/base_stats/).
+
+
+## NPCs that trade for a male Pokémon
+
+From [constants/npc_trade_constants.asm](../blob/master/constants/npc_trade_constants.asm):
+
+- `TRADE_GENDER_MALE`: A counterpart to `TRADE_GENDER_FEMALE`.
+
+Use it in [data/events/npc_trades.asm](../blob/master/data/events/npc_trades.asm).