code: public static final Atom add(Atom a, Atom b){
  
  		Atom c = new AtomImpl();
  		c.setX( a.getX() + b.getX() );
  		c.setY( a.getY() + b.getY() );
  		c.setZ( a.getZ() + b.getZ() );
  
  		return c ;
  	}
docs: add two atoms ( a + b).
  
  @param a
  an Atom object
  @param b
  an Atom object
  @return an Atom object