@article{SIGMA_2012_8_a16,
     author = {Johannes Tambornino},
     title = {Relational observables in gravity: a review},
     journal = {Symmetry, integrability and geometry: methods and applications},
     year = {2012},
     volume = {8},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SIGMA_2012_8_a16/}
}