\keepWithTag #'A {\keepWithTag #'B \music}

[Gianmaria Lari]

I thought that

  \keepWithTag #'(A B) \music

 and

  \keepWithTag #'A {\keepWithTag #'B \music}

would be equivalent. But according to the following test it doesn't.

1 Could you please confirm they are not the same?

2 Is it because \keepWithTag create a list of tags starting always
from the empty list?

* * *

The following code is a running example. It prints 4 staves: test A,
test B, test C, test D. I expected test A and test B would be
identical and also test C and test D.

\version "2.19.40"

note =
{
  \tag #'screenOut
    {
      \tag#'letter c'^\markup {M}
      \tag#'noLetter c'
    }
}

music =
{
  a \note
}

% test A
\score
{
  \keepWithTag #'(screenOut letter) \music
}
% test B
\score
{
  \keepWithTag #'screenOut {\keepWithTag #'letter \music}
}
% test C
\score
{
  \keepWithTag #'(screenOut noLetter) \music
}
% test D
\score
{
  \keepWithTag #'screenOut {\keepWithTag #'noLetter \music}
}