greek: Fix "faulty slice operation" for input ισαισα

The fix changes ισα to stem to ισ instead of the empty string, which
seems better (and to be what the second paper actually says to do if
read carefully).

Fixes #204

algorithms/greek.sbl

@@ -210,11 +210,11 @@ backwardmode (
   )
 
   define steps3 as (
+    (['{y}{s}{a}'] atlimit <- '{y}{s}') or
     [substring] among (
       '{y}{s}{a}' '{y}{s}{e}{s}' '{y}{s}{e}' '{y}{s}{a}{m}{e}' '{y}{s}{a}{t}{e}' '{y}{s}{a}{n}' '{y}{s}{a}{n}{e}' (
         delete
         unset test1
-        ('{y}{s}{a}' atlimit <- '{y}{s}') or
         ([] substring atlimit among (
           '{a}{n}{a}{m}{p}{a}' '{a}{th}{r}{o}' '{e}{m}{p}{a}' '{e}{s}{e}' '{e}{s}{oo}{k}{l}{e}' '{e}{p}{a}' '{x}{a}{n}{a}{p}{a}' '{e}{p}{e}' '{p}{e}{r}{y}{p}{a}'
           '{s}{u}{n}{a}{th}{r}{o}' '{d}{a}{n}{e}' '{k}{l}{e}' '{ch}{a}{r}{t}{o}{p}{a}' '{e}{x}{a}{r}{ch}{a}' '{m}{e}{t}{e}{p}{e}' '{a}{p}{o}{k}{l}{e}'