Single line comments 

Single line comments are added to an arbitrary tree as follows:

tree withComment("comments here", ...)

For example,

LIT(2) withComment("comments are useful",
  "only if they provide more info than the code")

prints as

// comments are useful
// only if they provide more info than the code

Scaladoc style comments 

Scaladoc style comments are written using withDoc as follows:

tree withDoc("comments" | doctag, ...)

where doctag is a doctag defined as DocTag.See(IntClass), DocTag.Author("Somebody"), etc.

For example,

(DEF("x") := LIT(0)) withDoc(
  "does something",

prints as

 * does something
 * @see scala.Int
def x = 0