Minecraft wiki entry for Item_modifier, associated text: set_enchantments - Modifies the item's enchantments.
 enchantments: Enchantments to modify.
 An enchantment: Key name is the enchantment ID. Value is a number provider specifying the enchantment level. Levels are clamped at a maximum of 255. Decreasing an enchantment level below 1 will remove the enchantment, through a relative negative add or through a direct value assignemnt.
 add: Optional. If true, change will be relative to current level. A nonexistent enchantment will be presumed to start at 0.