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M1.5 Spartan mapper can not fit design that fit in M1.4.
A case has been seen where the M1.5 mapper does not fit a
design that could be fin in M1.4. It does not appear to be
doing enough unrelated merging to fit the design.
M1.4 used 97/100 CLBs with "5 unrelated functions combined,
affecting 3 CLBs (3%)".
M1.5 used 101/100 CLBs with "One unrelated function packed
into a CLB".
The design does fit if the M1.4 results are used to guide mapping, so this is a possible work around.