must 'layer be "between 0 and 2"?

[Mark Polesky]

define-grob-properties.scm claims 'layer should be
"between 0 and 2", but other values clearly work.
Is there any reason why users should avoid other
values? I'm finding it *very* handy to use negative
values, for instance.

Perhaps it should be stated that no grob has a 
default value for 'layer below 0 or above 2, but 
that the user can set values outside that range.
This way, a user who sets only one 'layer value
over 2 is guaranteed to have that grob print over
everything else.

Is that correct, btw?

- Mark

\version "2.13.0"

{
  \once \override Staff.Clef          #'layer = #-2
  \once \override Staff.StaffSymbol   #'layer = #0
  \once \override Staff.TimeSignature #'layer = #4
  \once \override Staff.Stem          #'layer = #6

  \once \override Staff.Clef          #'color = #red
  \once \override Staff.TimeSignature #'color = #green
  \once \override Staff.Stem          #'color = #yellow
  c''4  
}

layers.png
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