code: public static double getAngle(AFPChain afpChain) throws StructureException {
  		if(afpChain.getBlockNum() < 1) {
  			throw new StructureException("No aligned residues");
  		}
  		Matrix rotation = afpChain.getBlockRotationMatrix()[0];
  
  		if(rotation == null) {
  			throw new NullPointerException("AFPChain does not contain a rotation matrix");
  		}
  		return getAngle(rotation);
  	}
docs: Calculate the rotation angle for a structure
  @param afpChain
  @return The rotation angle, in radians
  @throws StructureException If the alignment doesn't contain any blocks
  @throws NullPointerException If the alignment doesn't have a rotation matrix set