err:XS0022 & compound steps

Hi all,

The description of error err:XS0022 says:

"In all cases except the p:output of a compound step, it is a static
error if the port identified by a p:pipe is not in the readable ports of
the step that contains the p:pipe."

Perhaps I am just having a block here, but can somebody give me an
example why do we need to make exceptions for compound steps?

Thanks
Vojtech