Fix for +-rule-down

Argument presented in the source code was
  ;; note: We can't assert that X and Y are integers when Z is an integer since
  ;;       Z may be an integer when X and Y are Gaussian integers. But we can
  ;;       make such an assertion if either X or Y is real. If the Screamer
  ;;       type system could distinguish Gaussian integers from other complex
  ;;       numbers we could make such an assertion whenever either X or Y was
  ;;       not a Gaussian integer.
The argument above must surely be incorrect. If Z is an integer, and Y is a real
 say 2.5 then X will surely not be an integer. The argument *does* work if eithe
r X or Y are integers. So changed things so that Z being an integer and X OR Y being an INTEGER means that X AND Y are both integers. (Note the assertion is only for X, but this is correct given that the assertion for Y happens in another call to this rule.