R>#define unless(x) if (!(x))
[]

#define either(c)   ( ((x) ? 1 : 0)
#define or(c)       ^ ((y) ? 1 : 0) )

if either (x) or (y)
  // do something if one and only one of them is true
else
 // do something otherwise, i.e. if both are true or both are false