LazyUpdate::exec uses a &mut ref?

[juliusl]

Description

LazyUpdate has two exec fn's, exec and exec_mut. However it looks like their function signatures are identical?

https://github.com/amethyst/specs/blob/81073f340425803c99e3292fa14ceb00245bfea5/src/world/lazy.rs#L303-L305

https://github.com/amethyst/specs/blob/81073f340425803c99e3292fa14ceb00245bfea5/src/world/lazy.rs#L337-L339

I was wondering what is the difference between the two?