"Spin4 example"
"author : afmika"
"is-even program : takes an integer, prints 1 if it is even, 0 otherwise"

x                   "0"
(+0>)y              "1"
y                   "/!\ push an extra 1 to the stack : remove this part for an 'is-odd' program"
[,n]                "stack 0, 1, n"
[yx]+yx[yx]+[x]     "/!\ stack 0, 1, (n + 1) : /!\ remove this part for an 'is-odd' program"
(+0>)y[yx]xxy       "stack = 0, 1, (n + 1), (n + 1), 2 .. acc = (n + 1), 2"

{
    [yx]           "first iter -> stack 0, 1, (n + 1) .. acc = (n + 1), 2"
    /              "first iter -> stack 0, 1, (n + 1), ((n + 1) // 2) .. acc = (n + 1), 2"
    y              "first iter -> stack 0, 1, (n + 1), ((n + 1) // 2), 2 .. acc = (n + 1), 2"
    [>][>][>]      "first iter -> stack n, ((n + 1) // 2), 2, 0, 1"
    "increments the quotient q"
    [yx]
    +
    y
    [<][<][<]
?x}

"The stack should look like this 1, 1, (n + 1), q, 2"
"if n is even then (n + 1) = 2*q + 0"

[yx]            "acc = q, 2"
*               "stack = 1, 1, (n + 1), q * 2"
[yx]            "acc = (n + 1), q * 2"
-               "stack = 1, 1, r with r = (n + 1) - q * 2 = (n + 1) (mod 2)"
[.n]            "prints 0 if n is even, 1 otherwise"