y = -\frac12

\downarrow\downarrow\downarrow

\downarrow\downarrow\downarrow

\uparrow\uparrow\uparrow

\uparrow\uparrow\uparrow

f: y = -1 / 2
a(x, y) = (x > 0) ∧ (y > -0.5)
b(x, y) = (x  <  0) ∧ (y > -0.5)
c(x, y) = (x > 0) ∧ (y  <  -0.5)
d(x, y) = (x  <  0) ∧ (y  <  -0.5)