nested \relative ?

[Darren Ng]

I want to achieve the following:
(1) $a is respects $x as its "startpitch"
(2) $b is respects $x as its "startpitch"
(3) removing A has no effect on B

  \relative $x {
    %{A%} { $a ... }
    %{B%} { $b ... }
  }

In the example above: (1)=ok (2)=fail (3)=fail

  \relative $x {
    %{A%} \relative { $a ... }
    %{B%} \relative { $b ... }
  }

In the example above: (1)=fail (2)=fail (3)=ok

Idea #1:

  \relative $x {
    %{A%} { \backupCurrentRelativeOctave $a ... \restoreCurrentRelativeOctave }
    %{B%} { $b ... }
  }

Idea #2:

  \relative $x {
    %{A%} {
      c d e f g a b c d e f g a b c d e f g a b c
      % we are aware that the current relative octave has raised by 3
      % drop it back manually
      \ghostNoteWhichDoesNotActuallyAppear { c,,, }
    }
    %{B%} { $b ... }
  }

How to properly make B respect $x instead of the last note in A ?