Creating a type that is a non-contiguous subset of char?

[pmags]

One can exploit subranges to create a new type that is a subset of char, like 
so:
    
    
    type
        CapitalLetters = range['A'..'Z']
    
    var s = "The capital letters are: "
    for i in low(CapitalLetters)..high(CapitalLetters):
        s &= $i
    
    echo s
    
    
    Run

But is it possible to derive a type for a non-contiguous subset. Ideally 
something like this (non-working of course):
    
    
    type
        WASDLetters = set{'W','A','S','D'}
    
    
    Run