The classes of partially ordered groups
Compositio Mathematica, Tome 9 (1951), pp. 130-140. 
@article{CM_1951__9__130_0,
     author = {Loonstra, F.},
     title = {The classes of partially ordered groups},
     journal = {Compositio Mathematica},
     pages = {130--140},
     publisher = {Kraus Reprint},
     volume = {9},
     year = {1951},
     mrnumber = {45740},
     zbl = {0043.25602},
     language = {en},
     url = {http://archive.numdam.org/item/CM_1951__9__130_0/}
}
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Loonstra, F. The classes of partially ordered groups. Compositio Mathematica, Tome 9 (1951), pp. 130-140. http://archive.numdam.org/item/CM_1951__9__130_0/