docs: public MatrixParadigm initializeWithDistances(DistanceQuery<?> dq) {
      final DBIDArrayIter ix = this.ix, iy = this.iy;
      final double[] matrix = this.matrix;
      int pos = 0;
      for(ix.seek(0); ix.valid(); ix.advance()) {
        final int x = ix.getOffset();
        assert (pos == triangleSize(x));
        for(iy.seek(0); iy.getOffset() < x; iy.advance()) {
          matrix[pos++] = dq.distance(ix, iy);
        }
      }
      return this;
    }