code: public function setMapping( $severity, $pipe )
      {
          if ( !isset( $this->mapping[$severity] ) )
          {
              throw new \RuntimeException( "Unknown severity: " . $severity );
          }
  
          if ( ( $pipe !== self::SILENCE ) &&
               ( $pipe !== self::STDOUT ) &&
               ( $pipe !== self::STDERR ) )
          {
              throw new \RuntimeException( "Unknown output pipe: " . $pipe );
          }
  
          $this->mapping[$severity] = $pipe;
      }
docs: Set output pipe for severity
  
  Set the designated output pipe for log messages with the givene
  severity. The available severities are defined in the logger interface
  and are:
  
  - Logger::INFO
  - Logger::WARNING
  - Logger::ERROR
  
  The available output pipes are:
  
  - Cli::SILENCE, do not output anything
  - Cli::STDOUT, echo messages to STDOUT
  - Cli::STDERR, echo messages to STDERR
  
  @param int $severity
  @param int $pipe
  @return void