Operator overloading question

[Joseph Cassman]

Please refer to http://dpaste.dzfl.pl/edit/b73ef2cd

The code is contrived but is trying to focus on overloading the 
"+" and "*" operators for a struct.

Here is the output.

/home/c215/c527.d(36): Error: incompatible types for ((x) + (y)): 
'Arithmetic!(int)' and 'Arithmetic!(int)'
/home/c215/c527.d(37): Error: 'x' is not of arithmetic type, it 
is a Arithmetic!(int)
/home/c215/c527.d(37): Error: 'y' is not of arithmetic type, it 
is a Arithmetic!(int)

I am trying to understand why there are errors at all.
Also why the error for line 36 is different from the error for 
line 37.

Thanks

Joseph