@article{SF_1972__4_133915,
     author = {B. Tilson},
     title = {The intersection of free submonoids of a free monoid is free.},
     journal = {Semigroup forum},
     pages = {345--350},
     publisher = {mathdoc},
     volume = {4},
     year = {1972},
     zbl = {0261.20060},
     url = {http://geodesic.mathdoc.fr/item/SF_1972__4_133915/}
}