hello|
define split<1> as [
    set {} -> list|
    set '' -> word|
    set 0 -> i|
    do [
        do [
            set variable list + {variable word;} -> list|
            set '' -> word|
        ] else [
            set variable word + argument #1{variable i} -> word|
        ] if argument #1{variable i} = ' '|
        set variable i + 1 -> i|
    ] until variable i = do len<argument #1;>|
    do skip else set variable list + {variable word;} -> list if variable word = ''|
    return variable list|
]|

define is_number<1> as [
    set '0123456789' -> numbers|
    set 0 -> i|
    do [
        set 0 -> j|
        do [
            do skip else return false if argument #1{variable i} = variable numbers{variable j}|
            set variable j + 1 -> j|
        ] until variable j = do len<variable numbers;>|
        set variable i + 1 -> i|
    ] until variable i = do len<argument #1;>|
    return true|
]|

define calc<3> as [
    do return argument #2 + argument #3 if argument #1 = '+'|
    return 'error'|
]|

define unfold<1> as [
    do return 'error' if do len<argument #1;> = 0|

    set {} -> middle|
    set 1 -> i|
    do [
        set variable middle + {argument #1{variable i};} -> middle|
        set variable i + 1 -> i|
    ] until variable i = do len<argument #1;> - 1|

    do return 'error' if do len<variable middle;> = 0|

    do [
        set variable middle{0} -> middle|
        do set do int<variable middle;> -> n1 else return 'error' if do "is_number"<variable middle;>|
    ] else set do "unfold"<variable middle;> -> n1 if do len<variable middle;> = 1|

    do set do int<argument #1{do len<argument #1;>-1};> -> n2 else return 'error' if do "is_number"<argument #1{do len<argument #1;>-1};>|

    return do "calc"<argument #1{0}; variable n1; variable n2;>|
]|

set do input<':';> -> x|
set do "split"<variable x;> -> list|
do print<do "unfold"<variable list;>;>|
goodbye|