Thank you to Ole, and Question 1 (choice of colon operators)

[paralleluser]

Friends,

I want to begin by thanking Ole Tange for all his work and continuing efforts 
with GNU Parallel.  It is a very impressive set of work.  I've read over all 
the parallel docs, the man pages, the tutorials, the comparisons with other 
parallel running software, the ;login: article, and the parallel design choices 
document.  An amazing set of documents!  

I have a few questions of curiosity and hope you will allow me to post them 
over the next few days.  

Question 1:
The choice of ::: and :::: are unique.  Yes, they are arbitrary and they can be 
changed with parallel command line arguments.  But how/why where they chosen?  
I have a good guess as to why ":" and "::" were not used, but how did Ole 
decide on ::: and :::: and why did he skip : and :: ?

Much thanks....