@article{10_1017_S0017089500002317,
     author = {Mitchell, J. M. O.},
     title = {Counting word-trees},
     journal = {Glasgow mathematical journal},
     pages = {148--149},
     year = {1974},
     volume = {15},
     number = {2},
     doi = {10.1017/S0017089500002317},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089500002317/}
}