[css-grid] Clarifying "all lines in the implicit grid" text (which probably wants to exclude explicit grid)

Hi Tab,

I want to clarify the intended meaning of this chunk of the grid spec:
  # <integer> && <custom-ident>?
  #   [...]
  #   If not enough lines with that name exist,
  #   all lines in the implicit grid are assumed
  #   to have that name for the purpose of finding
  #   this position.
This language appears in 2 places:
  http://dev.w3.org/csswg/css-grid/#grid-placement-int
  http://dev.w3.org/csswg/css-grid/#grid-placement-span-int

Is the intention really that we should treat "all lines in the implicit
grid" as having the given name, in this scenario?  If I understand
correctly, "the implict grid" is basically *the whole grid*[1], so this
spec-text would require *every* line to match.  This seems like it'd
cause bizarrely discontinuous behavior when we cross into this "not
enough lines with that name" fallback case.

I think this spec text *really* means to say something like: "all lines
in the implicit grid, *aside from those in the explicit grid*, are
assumed to have that name".

However, that's not the meaning that I take away from the current
language, as explained below in [1].

(Looks like this spec-text was added by Tab here:
  https://hg.csswg.org/drafts/rev/14201#l1.25
Hence, I'm directing this question at Tab, though others are free to
comment as well of course. :))

~Daniel

[1] As I understand it, the "implicit grid" is effectively a superset of
the "explicit grid" -- at least, section 9.5 step 2 says that the
"number of columns in the implicit grid" is at least as large as "the
number of columns in the explicit grid".[2] This implies the implicit
grid is a superset of the explicit grid. And hence "all lines in the
implicit grid" (from my original quote) would include all lines in the
explicit grid.

[2] http://dev.w3.org/csswg/css-grid/#auto-placement-algo