In real life, pathfinding may require moving up or left rather than just right or down.  That's easy to add.  We just add left and up functions and include them in our choices:

function solve()
{
   while (!done())
      choose right(); or left(); or up(); or down();
}

function left() {
   map[x, y] = "left";
   x = x-1;
   if (x < 0 || map[x, y] != s) fail;
   map[x, y] = "green";
}

function up() {
   map[x, y] = "up";
   y = y-1;
   if (y < 0 || map[x, y] != s) fail;
   map[x, y] = "green";
}

This works, but it produces some decidedly peculiar paths.  This is because choose only promises to guess choices that lie on some solution path.  But that doesn't say that they are short or otherwise good solutions.  If you care which solution you get, then clairvoyance/nondeterminism by itself doesn't provide any optimality guarantees.

Try it out a few times:

// Find a way from the top-left corner to the bottom-right.
// Moving off the board or hitting an obstacle (occupied square) is failure.
function solve()
{
   while (!done())
      choose right(); or left(); or up(); or down();
}

// Design of the map
var s = "white";     // free space
var X = "red";       // obstacle
var size = 5;
var map = grid([
             [s, s, s, s, s],
             [s, s, s, s, s],
             [s, s, s, X, X],
             [s, s, s, s, s],
             [s, s, s, X, s]
           ]);

var x = 0;
var y = 0;

function right() {
   map[x, y] = "right";
   x = x+1;
   if (x == size || map[x, y] != s) fail;
   map[x, y] = "green";
}

function left() {
   map[x, y] = "left";
   x = x-1;
   if (x < 0 || map[x, y] != s) fail;
   map[x, y] = "green";
}

function up() {
   map[x, y] = "up";
   y = y-1;
   if (y < 0 || map[x, y] != s) fail;
   map[x, y] = "green";
}

function down() {
   map[x, y] = "down";
   y = y+1;
   if (y == size || map[x, y] != s) fail;
   map[x, y] = "green";
}

function done()
{
   return x == size-1 && y == size-1;
}

printLine("Starting map:");
printTilemap(map);
printLine();
solve();
printLine("Path:");
printTilemap(map);