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

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

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