I don't know the logical expression for .
I am thinking it could be one of these:
or
or
Could you help?
{x in B | x not in C} is the set of all x such that x is in B and x is not in C.
That is, for all x, we have x in {x in B | x not in C} if and only if (x is in B and x is not in C).
Put another way, {x in B | x not in C} = B\C, where '\' stands for relative complement.