Check for which of the following vector fields

            / f(x,y) \
  F(x,y) = |          |
            \ g(x,y) /

is the unit square  [0,1] x [0,1]  a "trapping region" (so that one may
attempt to apply Poincare-Bendixon).  

1. f(x,y) = y-x
   g(x,y) = x-y

2. f(x,y) = y(x-1)
   g(x,y) = -x

3. f(x,y) = x(y-1)
   g(x,y) = -y

4. f(x,y) = cos pi(x + y/8)

             2   2
   g(x,y) = x - y

5. f(x,y) = xy(x-1)
   g(x,y) = -xy-y

6. f(x,y) = xy(x-1)
   g(x,y) = -xy-1

You must check, for each case, in which directions the vector fields point at
each of the four sides of the uni square.