How do you safely deal with range.front?

[Tony]

For me, front() should throw a pre-defined exception when called 
on an empty range in order to eliminate undefined behavior. It 
does take some time to make a check, but D does array bounds 
checking by default. Ideally the front() check could be turned 
off somehow ("-boundschecks=off") by the user for those who want 
maximum speed, but I guess there is no way to do that when using 
pre-compiled functions in a library.