Rewrite example to use Retokenizer (resolves #3681)

Also add helper to filter spans

examples/information_extraction/entity_relations.py

@@ -36,11 +36,27 @@ def main(model="en_core_web_sm"):
             print("{:<10}\t{}\t{}".format(r1.text, r2.ent_type_, r2.text))
 
 
-def extract_currency_relations(doc):
-    # merge entities and noun chunks into one token
-    spans = list(doc.ents) + list(doc.noun_chunks)
+def filter_spans(spans, prefer_longest=True):
+    # Filter a sequence of spans so they don't contain overlaps
+    get_sort_key = lambda span: (span.end - span.start, span.start)
+    sorted_spans = sorted(spans, key=get_sort_key, reverse=prefer_longest)
+    result = []
+    seen_tokens = set()
     for span in spans:
-        span.merge()
+        if span.start not in seen_tokens and span.end - 1 not in seen_tokens:
+            result.append(span)
+            seen_tokens.update(range(span.start, span.end))
+    return result
+
+
+def extract_currency_relations(doc):
+    # Merge entities and noun chunks into one token
+    seen_tokens = set()
+    spans = list(doc.ents) + list(doc.noun_chunks)
+    spans = filter_spans(spans)
+    with doc.retokenize() as retokenizer:
+        for span in spans:
+            retokenizer.merge(span)
 
     relations = []
     for money in filter(lambda w: w.ent_type_ == "MONEY", doc):