pub trait Backend {
// Required method
fn trace(&self, args: BackendArgs) -> Result<(), TraceError>;
// Provided method
fn help(&self) -> Option<&str> { ... }
}pub trait Backend {
// Required method
fn trace(&self, args: BackendArgs) -> Result<(), TraceError>;
// Provided method
fn help(&self) -> Option<&str> { ... }
}