Difficult macro parameters - removing the implicit document tag

TL;DR: I'm trying to work out how get rid of the implicit document tag if I
press ENTER to insert a newline into a macro argument

This sample macro x can be used for the demo:

    <assign|x|<\macro|a|b|c>
      <arg|a> <arg|b><arg|c>
    </macro>>

If I invoke it like this:

  <x|aa|bb|cc>

I get the expected rendering:
aa bbcc

If I insert a new-line between the two a's, the source will invoke it like
this:

  <\x>
    a

    a
  </x|bb|cc>

And I get (as expected, due to implicit concatenation of document blocks):
a bbcc
a

If I delete the new-line separating the a's my source is:

  <\x>
    aa
  </x|bb|cc>

Although it renders as:
aa bbcc
this is NOT the same as before the newline:   <x|aa|bb|cc>

The difference is that the first argument now has label "document" but
before it had label "string" (or as it may have been in real life,
"concatenation" or "with" etc)

From within the editor, how do I get rid of the implicit document tag and
reduce back to string?

Sam