Preconditions are a way to validate state before running a command. They apply to all commands.

fun botPrecondition() = precondition {
    if (author.isBot)
        fail("Bots cannot do this!")

This code checks whether or not the user who attempted to invoke a command is a bot. If it is, the precondition will fail. All preconditions must be passed in order for a command to be run.

If a precondition is failed, the String passed into fail will be sent as a message in the channel where the command was attempted.