@article {VeVe,
AUTHOR = {Veli{\v{c}}kovi{\'c}, Boban and Venturi, Giorgio},
TITLE = {A direct proof of the five element basis theorem},
JOURNAL = {MLQ Math. Log. Q.},
FJOURNAL = {MLQ. Mathematical Logic Quarterly},
VOLUME = {63},
YEAR = {2017},
NUMBER = {3-4},
PAGES = {289--298},
}