Add GreekLemmatizer.lookup (resolves #4272)

spacy/lang/el/lemmatizer/__init__.py

@@ -46,6 +46,11 @@ class GreekLemmatizer(object):
         )
         return lemmas
 
+    def lookup(self, string):
+        if string in self.lookup_table:
+            return self.lookup_table[string]
+        return string
+
 
 def lemmatize(string, index, exceptions, rules):
     string = string.lower()

spacy/tests/regression/test_issue4272.py

@@ -0,0 +1,12 @@
+# coding: utf8
+from __future__ import unicode_literals
+
+from spacy.lang.el import Greek
+
+
+def test_issue4272():
+    """Test that lookup table can be accessed from Token.lemma if no POS tags
+    are available."""
+    nlp = Greek()
+    doc = nlp("Χθες")
+    assert doc[0].lemma_