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Hmm, you are correct. This is appears to be
an off-by-one problem: the original seed always gets
multiplied by the constant (which is silly), and the last
item in the tuple does not get multiplied (which causes
the bug).

The correct solution is to change:

value = my_mul(const, value) ^ hash(item)

in Steve's pseudo-code to:

value = my_mul(const, value ^ hash(item))

Of course, you still get a lot more robustness
for almost no cost if you vary "const" across
the tuple via a table.