diff --git a/setup.py b/setup.py
index 6f29e1efa..d579fd20e 100755
--- a/setup.py
+++ b/setup.py
@@ -40,6 +40,7 @@ MOD_NAMES = [
     "spacy.lexeme",
     "spacy.vocab",
     "spacy.attrs",
+    "spacy.kb",
     "spacy.morphology",
     "spacy.pipeline.pipes",
     "spacy.syntax.stateclass",
diff --git a/spacy/kb.pxd b/spacy/kb.pxd
index 92a0c8b95..43f3e83e8 100644
--- a/spacy/kb.pxd
+++ b/spacy/kb.pxd
@@ -4,6 +4,7 @@ from preshed.maps cimport PreshMap
 from libcpp.vector cimport vector
 from libc.stdint cimport int32_t, int64_t
 from .typedefs cimport hash_t
+from .strings cimport hash_string
 
 
 # Internal struct, for storage and disambiguation. This isn't what we return
@@ -32,10 +33,10 @@ cdef struct _EntryC:
 cdef struct _AliasC:
 
     # All entry candidates for this alias
-    const vector[int64_t] entry_indices
+    vector[int64_t] entry_indices
 
     # Prior probability P(entity|alias) - should sum up to (at most) 1.
-    const vector[float] probs
+    vector[float] probs
 
 
 cdef class KnowledgeBase:
@@ -94,13 +95,21 @@ cdef class KnowledgeBase:
                 feats_row=feats_row,
                 prob=prob
             ))
-        self._index[entity_key] = entity_index
+        self._entry_index[entity_key] = entity_index
         return entity_index
 
-    cdef inline int64_t c_add_aliases(self, hash_t alias_key, vector[int64_t] entry_indices, vector[float] probs):
+    cdef inline int64_t c_add_aliases(self, hash_t alias_key, entities, probabilities):
         """Connect a mention to a list of potential entities with their prior probabilities ."""
         cdef int64_t alias_index = self._aliases_table.size()
 
+        cdef vector[int64_t] entry_indices
+        cdef vector[float] probs
+
+        for entity, prob in zip(entities, probs):
+            entry_index = self._entry_index[hash_string(entity)]
+            entry_indices.push_back(entry_index)
+            probs.push_back(prob)
+
         self._aliases_table.push_back(
             _AliasC(
                 entry_indices=entry_indices,
diff --git a/spacy/kb.pyx b/spacy/kb.pyx
index 0f6a7aecc..d2b8fffe1 100644
--- a/spacy/kb.pyx
+++ b/spacy/kb.pyx
@@ -1,34 +1,42 @@
-from .strings cimport hash_string
+# cython: profile=True
+# coding: utf8
+from preshed.maps import PreshMap
 
 
 cdef class KnowledgeBase:
+
+    def __init__(self):
+        self._entry_index = PreshMap()
+        self._alias_index = PreshMap()
+        self.mem = Pool()
+
+
     def __len__(self):
         return self._entries.size()
 
-    def add_entity(self, entity_id: str, float prob, vectors=None, features=None):
+    def add_entity(self, unicode entity_id, float prob, vectors=None, features=None):
+        cdef hash_t id_hash = hash_string(entity_id)
+
         # TODO: more friendly check for non-unique name
-        if entity_id in self:
+        if id_hash in self._entry_index:
             return
 
-        cdef hash_t id_hash = hash_string(entity_id)
+
         cdef int32_t dummy_value = 342
         self.c_add_entity(entity_key=id_hash, prob=prob, vector_rows=&dummy_value, feats_row=dummy_value)
         # TODO self._vectors_table.get_pointer(vectors),
         # self._features_table.get(features))
 
-    def add_alias(self, alias, entities, probabilities):
+    def add_alias(self, unicode alias, entities, probabilities):
         """For a given alias, add its potential entities and prior probabilies to the KB."""
         cdef hash_t alias_hash = hash_string(alias)
-        cdef hash_t entity_hash = 0
-        cdef int64_t entity_index = 0
-
-        cdef vector[int64_t] entry_indices = [self._entry_index[hash_string(entity)] for entity in entities]
-
-        self.c_add_aliases(alias_key=alias_hash, entry_indices=entry_indices, probs=probabilities)
 
         # TODO: check that alias hadn't been defined before
         # TODO: check that entity is already in this KB (entity_index is OK)
         # TODO: check sum(probabilities) <= 1
         # TODO: check len(entities) == len(probabilities)
 
+        self.c_add_aliases(alias_key=alias_hash, entities=entities, probabilities=probabilities)
+
+
 
diff --git a/sandbox_test_sofie/__init__.py b/spacy/sandbox_test_sofie/__init__.py
similarity index 100%
rename from sandbox_test_sofie/__init__.py
rename to spacy/sandbox_test_sofie/__init__.py
diff --git a/sandbox_test_sofie/testing_el.py b/spacy/sandbox_test_sofie/testing_el.py
similarity index 67%
rename from sandbox_test_sofie/testing_el.py
rename to spacy/sandbox_test_sofie/testing_el.py
index 7883e44d4..840d890b5 100644
--- a/sandbox_test_sofie/testing_el.py
+++ b/spacy/sandbox_test_sofie/testing_el.py
@@ -1,4 +1,16 @@
 import spacy
+from spacy.kb import KnowledgeBase
+
+
+def create_kb():
+    mykb = KnowledgeBase()
+    print("kb size", len(mykb))
+
+    entity_id = "Q42"
+    mykb.add_entity(entity_id=entity_id, prob=0.5)
+    print("adding entity", entity_id)
+
+    print("kb size", len(mykb))
 
 
 def add_el():
@@ -23,4 +35,5 @@ def add_el():
 
 
 if __name__ == "__main__":
-    add_el()
+    # add_el()
+    create_kb()