hjson is an actively used metalanguage created in 2014.

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Example code from the web:

{
 # hash style comments
 # (because it's just one character)

 // line style comments
 // (because it's like C/JavaScript/...)

 /* block style comments because
    it allows you to comment out a block */

 # Everything you do in comments,
 # stays in comments ;-}
}
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Last updated February 11th, 2019