Address special unicode sigma at end of term when lowercasing.

Σ maps to σ, except at the end of a word where it maps to ς.
This is the only conditional (contextual) but language-independent
mapping in unicode.

analysis/token/lowercase/lowercase.go

@@ -75,6 +75,13 @@ func toLowerDeferredCopy(s []byte) []byte {
 			continue
 		}
 
+		// Handles the Unicode edge-case where the last
+		// rune in a word on the greek Σ needs to be converted
+		// differently.
+		if l == 'σ' && i + 2 == len(s) {
+			l = 'ς'
+		}
+
   		lwid := utf8.RuneLen(l)
 		if lwid > wid {
 			// utf-8 encoded replacement is wider

analysis/token/lowercase/lowercase_test.go

@@ -48,6 +48,9 @@ func TestLowerCaseFilter(t *testing.T) {
 		&analysis.Token{
 			Term: []byte("ȺȾCAT"),
 		},
+		&analysis.Token{
+			Term: []byte("ὈΔΥΣΣ"),
+		},
 	}
 
 	expectedTokenStream := analysis.TokenStream{
@@ -69,6 +72,9 @@ func TestLowerCaseFilter(t *testing.T) {
 		&analysis.Token{
 			Term: []byte("ⱥⱦcat"),
 		},
+		&analysis.Token{
+			Term: []byte("ὀδυσς"),
+		},
 	}
 
 	filter := NewLowerCaseFilter()