expander: $_ most recent parameter

[marimlem]

$_ most recent parameter (or the abs path of the command to start the current shell immediately after startup).

[marimlem]

• [x] find out if it's needed

if implemented, needs to be accompanied with check, that char after _

1. exists (could be $_\0, as to prevent errors in following checks)
2. is not digit (if it is digit then the variable is $_1 and not $_)
3. is not char (if it is char then the variable is $_a and not $_)

[marimlem]

not required by subject