chore: import upstream snapshot with attribution

examples/lua/symmetric_difference.lua

@@ -0,0 +1,20 @@
+#!/usr/bin/env luajit
+A = { ["John"] = true, ["Bob"] = true, ["Mary"] = true, ["Elena"] = true }
+B = { ["Jim"] = true, ["Mary"] = true, ["John"] = true, ["Bob"] = true }
+
+A_B = {}
+for a in pairs(A) do
+    if not B[a] then A_B[a] = true end
+end
+
+B_A = {}
+for b in pairs(B) do
+    if not A[b] then B_A[b] = true end
+end
+
+for a_b in pairs(A_B) do
+    print( a_b )
+end
+for b_a in pairs(B_A) do
+    print( b_a )
+end