Code improvement for DNA reverse complement?

[biocyberman via Digitalmars-d-learn]

I am solving this problem http://rosalind.info/problems/revc/ as 
an exercise to learn D. This is my solution:

https://dpaste.dzfl.pl/8aa667f962b7

Is there some D tricks I can use to make the `reverseComplement` 
function more concise and speedy? Any other comments for 
improvement of the whole solution are also much appreciated.