min:   2 x1  -  x2  +5 x3 +3 x4;
c1:    7 x1         -3 x3 +2 x4  = 23;
c2:   -3 x1  +6 x2  +2 x3 +5 x4 <= 18;
c3:    4 x1  -  x2        +2 x4 >= 12;

	-inf < x1 < +inf;
//	0 <= x2 < +inf;
//	0 <= x3 < +inf;
	-inf < x4 <= 0;