Some basic promise types
------------------------

files
    A promise about a file, including its existence, attributes and contents.

packages
    A promise to install (or remove or update or verify) a package.

processes
    A promise concerning items in the system process table (such as
    that a process is present or absent).

vars
    A promise to be a variable, representing a value.

reports
    A promise to report a message.

commands
    A promise to execute a command.
