class SyntaxTree::YARV::BasicBlock

def each_with_length

original instruction sequence.
Yield each instruction in this basic block along with its index from the 
def each_with_length
  return enum_for(:each_with_length) unless block_given?
  length = block_start
  insns.each do |insn|
    yield insn, length
    length += insn.length
  end
end