Immutability and other attributes, please review

[Roman D. Boiko]

I have not much experience in D programming yet, and one of 
things which I frequently mess up is usage of various attributes, 
especially immutable and related.

Could anybody make a review my code at 
https://github.com/roman-d-boiko/functional-data-structures/blob/master/fds/persistent.d, 
https://github.com/roman-d-boiko/dct/blob/master/fe/syntax.d, and 
https://github.com/roman-d-boiko/dct/blob/master/fe/_/syntax.d 
with respect to usage of attributes (immutable, const, pure, 
etc.), contracts, and any other issues.

I'm redesigning my previous coding attempts to build D compiler 
front end, and would like to pay attention to these aspects. 
Please note that adding DDoc comments has been deferred because I 
need to learn how to use them.