code: public synchronized Vector getVector(String word) {
          Vector vector = wordToVector.get(word);
          if (vector != null)
              return Vectors.immutable(vector);
  
          Vector v = backingSpace.getVector(word);
          if (v != null)
              wordToVector.put(word, v);
          return v;
      }
docs: {@inheritDoc} If the word is in the semantic space, its vector will be
  temporarily loaded into memory so that subsequent calls will not need to
  go to disk.  As memory pressure increases, the vector will be discarded.
  
  @throws IOError if any {@code IOException} occurs when reading the data
  from the underlying semantic space file.