Macro trace 

macro_rules! trace {
    (parent: $parent:expr, $format:expr $(, $($path:ident).+ $(= $value:expr)?)* $(,)?) => { ... };
    ($format:expr $(, $($path:ident).+ $(= $value:expr)?)* $(,)?) => { ... };
}

Emit a log at the TRACE level.

See the log! macro for more details.