Exercises for Procedures and Modules
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1. Write a program that calls a funtion to sum all of the integers
   between min and max.  Set min and max to be optional keyword
   arguments which default to 1 and 10 respectively.  (opt_par.f90)

2. Look at program err_main.f90 and err_sub.f90. Compile and run.
   What is wrong?  Rewrite in a better way in Fortran 90.

3. Write a recursive function to calculate the nth value of the
   Fibonacci sequence.  Notice that fib(1) = 1, fib(2) = 1, fib(i) =
   fib(i-1) + fib(i-2) i.e.  1, 1, 2, 3, 5, 8, 13, ...  (fibon.f90)

4. Write a program which defines a generic function to return the
   maximum absolute value of two variables, for real, integer and
   complex variable types.  (maxabs.f90)

5. Write a module which defines kind values for single and double
   precision real variables, and a main program which uses this module
   and can be changed from single to double precision by changing a
   single value.  (prec.f90, prec_mod.f90)

6. Look at the program generic.f90.  Modify this program to include a
   function for swapping two variables of type (point) by using module, 
   where 'point' is defined with two real variables.  
   (gen_mod.f90, swap_mod.f90)

7. Look at the program money.f90.  From these code fragments, construct
   a module that allows you to run the program mon_main.f90.
   (mon_main.f90, mon_mod.f90)

8. Write a module which defines a vector type with x and y components
   and the associated operators '+' and '-' overloading, and a main program
   which uses this module to apply all associated operators overloading 
   to the variables of derived type vector. 
   (vec_main.f90, vec_mod.f90)