We write:
$"Natural numbers".zero
$"Natural numbers".successor(n = $number(value = 0))
$"Natural numbers".successor(n = $number(value = 1))
$"Natural numbers".successor(n = $number(value = 2))
Positional notation is implemented as a macro, i.e. using additional code that is injected into the proof checker. Therefore, no definition is given here.