2 class issues

[spir]

Hello,

First, I am not very experimented with the combination of static lang (alloc & 
typing) and OO (class-based). I'm implementing a library for lexical analysis 
(lexing), with 2 minor issues:

-1- How to enforce that subclasses implement given methods without using 
"abstract", which seems to make the whole class abstract? (no info found in doc, 
actually, the page on classes [1] does not seem to even mention abstract classes)

-2- How to have "constant" (predefined) class instances at the module-level? The 
compiler requires a "static this ()". What does this actually mean (for a 
constructor)? What are the consequences, for either my code or client code? (The 
doc on the topic [2] is rather obscure for me, and I could not find better 
elsewhere.)

I'm also bluffed by "Static constructors have empty parameter lists." Does this 
mean I should manually fill the fields? (not a big deal, but why???) This may give:
     // Predefined pseudo-pattern "End-of-Text":
     auto EoT = new Pattern() ;       // ???
     EoT.name = "EoT" ;

     // Unique lexeme "end-of-text":
     auto eot = new Lexeme() ;       // ???
     eot.patname = "EoT" ;
     eot.slice = null ;
     eot.index = uint.max ;
Then, why have a constructor at all? This would also prevent me from making 
classes immutable, while conceptually all are immutable... (no reason for a 
pattern or a lexeme to change)

Thank you,
diniz

[1] https://dlang.org/spec/class.html
[2] https://dlang.org/spec/class.html#static-constructor