Substitute a command into another command?

[LightAndLight]

Example:

f : (| Some : String, None : () |) -> Cmd
f b? =
  let command = `echo a` in
  case b? of
    None () ->
      command
    Some b ->
      `$command $b`

repl> f None
`echo a`

repl> f (Some "b")
`echo a b`

We could allow substitution of a Cmd into the first item of another Cmd. Right now you can only substitute a String in this position:

repl> :type \x -> `$x a b c`
String -> Cmd

This idea occurred to me when trying to "optionally" pass an argument to a command. I can't do something like echo a ${case b? of; None () -> ""; Some b -> b}, because that passes always passes the second argument (which has the value of "" when b? = None()).