code coverage tests:

1. int
    $s[1]
    $i(^s.int[])
    1.1 ^i.double(2)
    1.2 ^i.dec(1) $i
    1.3 ^i.inc(1) $i
    1.4 ^i.mul(2) $i

2. double
    $d(1.1)
    2.1 ^d.double(2)

3. bool
    $b(true)
    3.1 ^b.double(2)
    3.2 ^if(^b.bool(false)){true}{false}