A contour is a path in the complex plane over which contour integration is performed to compute
a contour integral. When choosing a contour to
evaluate an integral on the real line, a contour is
generally chosen based on the range of integration and the position of poles
in the complex plane. For example, for an integral
from 
to 
along the real axis, the contour at left could be chosen
if the function 
had no poles on the real line,
and the middle contour could be chosen if it had a pole
at the origin. To perform an integral over the positive real axis from 0 to 
for a function with a pole
at 0, the contour at right could be chosen.
