test, provide regression cases for recursive summing with and without 0 ratios

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
fix, allow composition of 0/0 ratios (treat them as 1/1)

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'core/caev-arith'

* core/caev-arith:
  fix, allow composition of 0/0 ratios (treat them as 1/1)
  test, provide regression cases for recursive summing with and without 0 ratios