docs: function mean( out, x ) {
  	var len = x.length,
  		i;
  	if ( out.length !== len ) {
  		throw new Error( 'mean()::invalid input arguments. Input and output matrices must be the same length.' );
  	}
  	for ( i = 0; i < len; i++ ) {
  		out.data[ i ] = MEAN( x.data[ i ] );
  	}
  	return out;
  }