! Nemhauser & Wolsey, 2. "B&B using LP relaxations"
! Exa. 2.1, p 360 z* = 30.18, X=[36 49 0 0 75]/11 = 3.27, 3.63, etc.
! Branch on x2 (3, 4): z = 29.8, X=[3.4, 3], z = infeasible
! Branch on x1 (3, 4): z = 27,   X=[3,   3], z_MAX = 28, X=[4, 0]
Maximize 7 x1 + 2 x2
st
 -x1 + 2 x2 < 4
 5x1 + 1 x2 < 20
-2x1 - 2 x2 < -7
! x2 < 3
! x1 > 4
end