public static IEnumerable <string> GetFinites(this string[,] grid)
{
Guard.Argument(() => grid).NotNull().NotEmpty();
var size = grid.GetSize();
var maxX = size.Width - 1;
var maxY = size.Height - 1;
var labels = (from x in Enumerable.Range(0, size.Width)
from y in Enumerable.Range(0, size.Height)
let label = grid[x, y]
where label != "."
orderby label
select label.ToLowerInvariant()
).Distinct();
var infinites = (from label in grid.GetEdgeValues()
where label != "."
orderby label
select label.ToLowerInvariant()
).Distinct();
var finites = labels.Except(infinites).OrderBy(s => s);
return(finites);
}