(sql490.htm), there is

"set of all k-tuples v1, v2, ... vk, such that v1 [isin] D1, v1 [isin] D1"

i assume it should be
"set of all k-tuples v1, v2, ... vk, such that v1 [isin] D1, v2 [isin] D2"

Divide by Zero