how did a normal function to be converted to bevy 'system'.

[cqlangyi]

with bevy 0.7

App::new()
.add_system(func1.before(func2))
...

with above code snippet, 'func1' be converted into a bevy 'system' automatically.

functions like 'before()', 'after()'... etc. they seem to be appended on to 'func1' some how, but don't know where from?

my question is: how did that happened?

any hint would be appreciated.

[tim-blackbird]

These methods come from an extension trait that is exported from the bevy::prelude.

[cqlangyi]

big thank for your answer. just what i'm expecting.