Question on technique in musicals

[Francesca]

There's a phenomenon whereby the songs in a musical reflect one another. That is, you can hear the melodies of other songs in the songs. For example, in Sweeney Todd, the tune for "Pirelli's Miracle Elixir" is very similar to that for "God, That's Good". Sondheim does this a lot, and I can quite often hear three or four different songs reflected in one song, like threads. I'm thinking there must be a name for it. Any help?