docs: function l1norm( arr ) {
  		if ( !Array.isArray( arr ) ) {
  			throw new TypeError( 'l1norm()::invalid input argument. Must provide an array.' );
  		}
  		var len = arr.length,
  			sum = 0,
  			val;
  		for ( var i = 0; i < len; i++ ) {
  			val = arr[ i ];
  			// Math.abs( val )
  			if ( val < 0 ) {
  				val = -val;
  			}
  			sum += val;
  		}
  		return sum;
  	}