If f(x) = x2,
g(x) = x – 1,

Pickling is the process that uses a vinegar solution to preserve foods.

Of that are from eating black  foods or iron supplements. the most common condition causing  it is a bleeding ulcer.  black poop caused by blood indicate a problem in the upper digestive tract.     along with pain, vomiting, or diarrhea is cause to see a doctor right go see a doctor