Silence as long as another part

[Joel C. Salomon]

Neighbors,

How can I set one piece to be silent for the duration of another?  I
know I’ve seen this in the past, but I cannot find it again.

E.g.,

    upper = { c1 d e f }

    dynamics = {
      <>\p
      s1*4 % <-- this line: silence as long as \upper
      \bar "|."
    }

(This is obviously more useful in larger pieces; e.g., where the first
sub-melody and the third have dynamics changes or pedal movements and
the second does not.)

––Joel C. Salomon