anchor.diff:  
   %% Final challenge: functions are first class citizens (should be no surprise!).
   %% Write the higher-order functions map, filter, and reduce.
  +
  +
  
positive.code: 
  demo_quadratic() ->
    A = 1,
    B = 2,
    C = 1,
    case quadratic_solve(A, B, C) of
      {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.
  
  
  
  
negative.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,
  filter/2]).
  
  %% You include the "arity" of the function (how many arguments it takes)
  %% when referring to it.
  