In /usr/share/X11/xkb/symbols/gr, the "simple" variant of Greek allows
for apostrophe and double-quote on line 78:

key <AC11> { [   apostrophe,    quotedbl ] }; // ' "

However, this is overridden in polytonic Greek (the "polytonic"
variant) by the necessary dead keys:

key <AC11> { [ dead_grave, dead_dasia  ] }; // ` ̔

In polytonic ancient (and perhaps modern) Greek, phrases such as "οἶός
τ' εἰμί" (I am able) are common, and it is a pain to have to change
keyboards for just one character every so often.

It would be nice to be able to have these characters available in the
standard polytonic Greek keyboard.  One possibility is to have key
<AC11> be a FOUR_LEVEL key, allowing ' (apostrophe) using right-alt+'
and " (quotedbl) using right-alt+shift+'.  Here is a tested patch
which achieves this:

--- gr.orig     2012-12-17 00:00:45.592885801 +0000
+++ gr  2012-12-17 00:13:38.536165180 +0000
@@ -180,7 +180,9 @@
     key <AD12> { [  dead_iota,     VoidSymbol,  dead_breve ] }; // ͺ   ˘

     key <AC10> { [ dead_acute, dead_psili  ] }; // ´ ̓
-    key <AC11> { [ dead_grave, dead_dasia  ] }; // ` ̔
+    key <AC11> { type[Group1]="FOUR_LEVEL",
+                 [ dead_grave, dead_dasia, apostrophe, quotedbl  ] };
+                 // ` ̔  ' "
 };

 partial alphanumeric_keys alternate_group


This seems to be the only significant functionality which is lost in
the polytonic keyboard; the braces and brackets which are also lost
are far less important, AFAICT, and there is no obvious place to
reintroduce them (as the third level is already used for these keys in
the polytonic keyboard).

   Julian