Given three numbers x, y, z, determine if one of them is the sum of the other two.
If so, print the equation, otherwise print "None found". For example, if x = 2,
y = 1, and z = 3, print "z = x + y". If there are multiple possibilities (such as
all values equal to 0) favor the solutions in this order: "x = y + z", "y = x + z",
and "z = x + y".