code: func opcodeDepth(op *parsedOpcode, vm *Engine) error {
  	vm.dstack.PushInt(scriptNum(vm.dstack.Depth()))
  	return nil
  }
docs: // opcodeDepth pushes the depth of the data stack prior to executing this
  // opcode, encoded as a number, onto the data stack.
  //
  // Stack transformation: [...] -> [... <num of items on the stack>]
  // Example with 2 items: [x1 x2] -> [x1 x2 2]
  // Example with 3 items: [x1 x2 x3] -> [x1 x2 x3 3]