docs: function rad2deg( x ) {
  	var isArray = Array.isArray( x ),
  		len;
  	if ( !isArray && ( typeof x !== 'number' || x !== x ) ) {
  		throw new TypeError( 'rad2deg()::invalid input argument. Must provide either a single numeric value or a numeric array.' );
  	}
  	if ( !isArray ) {
  		return x * 180 / Math.PI;
  	}
  	len = x.length;
  	if ( !len ) {
  		return null;
  	}
  	for ( var i = 0; i < len; i++ ) {
  		x[ i ] *= 180 / Math.PI;
  	}
  	return x;
  }