anchor.diff:  
   demo_quadratic() ->
     A = 1,
  -  B = 2,
  +  B = 0,
     C = 1,
     case quadratic_solve(A, B, C) of
  +    undefined -> io:format("Zero real solutions");
       {X1, X2} -> io:format("Two solutions: ~f and ~f~n", [X1, X2]);
       X -> io:format("One solution: ~f~n", [X])
     end.
   
   %% Challenge: write reverse, which reverses a list.
  +
   
   
   
  
positive.code: %% unless you export them.
  -module(learning).
  -export([first/1, next/1, add/2, print_and_add/2, is_even/1, newfirst/1, second/1, same/2,
  abs/1, appropriate_age_for_mlp/1, beach_weather/1, fib/1, len/1, quadratic_solve/3, demo_quadratic/0]).
  
  %% You include the "arity" of the function (how many arguments it takes)
  %% when referring to it.
  
negative.code: 
  %% Final challenge: functions are first class citizens (should be no surprise!).
  %% Write the higher-order functions map, filter, and reduce.
  
  
  