code: func (b *BlockGen) PrevBlock(index int) *types.Block {
  	if index >= b.i {
  		panic(fmt.Errorf("block index %d out of range (%d,%d)", index, -1, b.i))
  	}
  	if index == -1 {
  		return b.parent
  	}
  	return b.chain[index]
  }
docs: // PrevBlock returns a previously generated block by number. It panics if
  // num is greater or equal to the number of the block being generated.
  // For index -1, PrevBlock returns the parent block given to GenerateChain.