Macro define_compression_level 

macro_rules! define_compression_level {
    (
        $name:ident,
        Min => $min:expr,
        Max => $max:expr,
        Default => $default:expr,
        $compression:literal,
        $url:literal
    ) => { ... };
}

A macro to define a compression level struct.

Accepts the name of the compression level struct, its min, max and default values, the compression executable it relates to and a url, that defines a man page for the compression executable.