That is conventional set subtraction notation. "Assign to X the same set minus all elements of the set {a_i}". One example source, but it is pretty common in g

That is conventional set subtraction notation. "Assign to X the same set minus all elements of the set {a_i}". One example source, but it is pretty common in general: http://www.mathwords.com/s/set_subtraction.htm http://www.mathwords.com/s/set_subtraction.htm