* Replaces definition of a builtin hud element
* `name`: `"breath"` or `"health"`
* `hud_definition`: definition to replace builtin definition
+* `minetest.parse_relative_number(arg, relative_to)`: returns number or nil
+ * Helper function for chat commands.
+ * For parsing an optionally relative number of a chat command
+ parameter, using the chat command tilde notation.
+ * `arg`: String snippet containing the number; possible values:
+ * `"<number>"`: return as number
+ * `"~<number>"`: return `relative_to + <number>`
+ * `"~"`: return `relative_to`
+ * Anything else will return `nil`
+ * `relative_to`: Number to which the `arg` number might be relative to
+ * Examples:
+ * `minetest.parse_relative_number("5", 10)` returns 5
+ * `minetest.parse_relative_number("~5", 10)` returns 15
+ * `minetest.parse_relative_number("~", 10)` returns 10
* `minetest.send_join_message(player_name)`
* This function can be overridden by mods to change the join message.
* `minetest.send_leave_message(player_name, timed_out)`