alias this

[Carl Sturtivant]

Subtyping with 'alias this' is superb and easy; it's a tremendous 
simplifying mechanism, one of D's many ingenious and well 
integrated technical simplifying mechanisms that were a big part 
of getting me on board, spreading the word and moving to D. What 
is the design logic behind

364.d(9): Error: alias this compileme364.number.__anonymous there 
can be only one alias this

--- not a complaint, I'd just like to hear opinion of the basis 
of the above restriction from experts.