Xor

Takes the two top values on the boolean stack and xor's them together. If one is true and one is false it places true on the stack. If both are true, or if both are false, it places false on the stack.

false false and    puts false on the stack
true false and     puts true on the stack
false true and     puts true on the stack
true true and      puts false on the stack

Logic: true false and or not xor