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!

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Regrads,
Japin Li