For emacs snippet, how to expand another snippet inside one snippet

Question

I want to expand another snippet which is predefined into the current template file, how can I realize it?
For example:
Say I already have a snippet called 'snippet1', and I hope I can write snippet like this:

# -*- mode: snippet -*-
# name: snippet2
# key: snippet2
# --
`(expand 'snippet1)`
blablabla......

The string (expand 'snippet1) is just an example.
How can it be achieved?