Search zone
=============

#. Write a function *distance* that takes four numbers
   that contain the coordinates of two points 
   (x1,y1) and (x2,y2), and returns
   the `distance <https://en.wikipedia.org/wiki/Distance>`_
   between those two points.
   
#. Write a function *circumference_area* that takes the
   radius of a circumference and returns its area.

#. A mountain rescue team needs an application to compute
   the area of a given zone. Sometimes they are given two
   reference points to determine a circular search zone, the
   first one being the center and the second lying on the
   perimeter.
   Write the function *compute_search_area* that takes
   the x,y coordinates of the center and the x,y
   coordinates of a point on the perimeter, and returns
   the area of the search zone. This function will call
   the two functions on the preceding paragraphs.