in

in for, iterate over a specified list, map or another container:
```
for $host in [1, 2, 3] {
    print ($number ** 2)
}
```

a binary operator that checks an element (left argument) exists in a container (right argument).

It has the same priority as comparison operations like <= or > and higher priority than logical operations and or or. All other operations are higher priority.

check that item exists in a list

$list := [1, 2, [4], 6]

put (1   in $list) # true
put ([4] in $list) # true
put (4   in $list) # false

check key exists in a dictionary

$languages := {
    python: null,
    rust:   null,
    go:     null,
}
$hasC  := "c"  in $map # false
$hasGo := "go" in $map # true

check if $age is a teenage-range of 13..18 inclusive.

if $age in (range 13 19) {
    print "$age yo is a teenager"
}