[m-rev.] for review: better lookahead for switch detection

[Zoltan Somogyi]

This is the diff I was talking about on m-dev. For review by anyone.

The attached y.m shows the effect of the diff on an extract from
compiler/print_help.m. Without this diff, switch detection cannot handle it;
with the diff, it can handle it.

This diff does not implement either fix one, two or four discussed on m-dev.
Instead, it sidesteps the problem (the diff to message.m does this).

After this diff is committed, I intend to implement fix four. The attached z.m,
which is the simplified essence of the problem with the old message.m,
is intended to become the test case for that fix (under a more descriptive name,
of course). It will need to be extended to test the fix for Peter's example code
as well.

Zoltan.
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