Returns
            the greatest common denominator of two integers
         
         
         Arguments
         
- int1
 
            - 
               
                  An integer; must be greater
                     than 0.
                  
                
             - int2
 
            - 
               
                  An integer; must be greater
                     than 0.
                  
                
             
         
         Return Values
         
         An integer representing
            the greatest common denominator between int1 and int2.
         
         Examples
         
         Command: (gcd
               81 57)
         3
         Command: (gcd
               12 20)
         4