// A hypothetical scanner class
struct _Scan {
    int !x;
    u8[~] buf;
    new _Scan={ u8[~] b -> buf=b; };
    // Skip whitespace
    val skip = { ->
        while( buf[x] <= ' ' )
            x++;
    };
    val require = { u8 ch ->
        skip();
        buf[x++]==ch;
    };
};
!_Scan !s = new _Scan("  [1,2]");
return s.require('[');