linear programming - How to formulate x != y in lpsolve? -

i'm trying formulate variables x,y,z must different , accept values 1, 2 or 3 (this is, of course, toy example):

min: x+y+z;  1 <= x <= 3; 1 <= y <= 3; 1 <= z <= 3; 

but make work still need either access boolean operators or != operator, don't seem exist in lpsolve! how can go this? wanted this:

x != y; x != z; y != z; 

thanks

edit:

here's current code:

/* objective function */ min: 1;  /* variable bounds */ 1 <= x1 <= 4; 1 <= x2 <= 4; 1 <= x3 <= 4; 1 <= x4 <= 4;   x1 + x2 + x3 + x4 = 10;  x1 < x2; x2 < x3; x3 < x4;  int x1; int x2; int x3; int x4; 

lpsolve giving me result:

x1 = 1 x2 = 3 x3 = 3 x4 = 3 

which wrong. why?