MATH PUZZLE: Can you replace...

A man placed some flowers on the grave of his dearly departed mother and started back toward his car when his attention was diverted to another man kneeling at a grave. The man seemed to be praying with profound intensity and kept repeating, "Why did you have to die? Why did you have to die?"
The first man approached him and said, "Sir, I don't wish to interfere with your private grief, but this demonstration of pain is more than I've ever seen before. For whom do you mourn so deeply? A child? A parent?"
The mourner took a moment to collect himself, then replied, "My wife's first husband."