Confused about gram.y referencs in Makefile?

Hi, hackers

I find src/backend/utils/mb/Unicode/Makefile has the following comments:

> # Note that while each script call produces two output files, to be
> # parallel-make safe we need to split this into two rules.  (See for
> # example gram.y for more explanation.)
> #

I could not find the explanation in gram.y easily.  Would someone point
it out for me?  Thanks in advance!

-- 
Regrads,
Japin Li