for X, Y being Subset of TarskiEuclid2Space st ex a being Element of TarskiEuclid2Space st
for x, y being Element of TarskiEuclid2Space st x in X & y in Y holds
between a,x,y holds
ex b being Element of TarskiEuclid2Space st
for x, y being Element of TarskiEuclid2Space st x in X & y in Y holds
between x,b,y