code: func opcodeElse(op *parsedOpcode, vm *Engine) error {
  	if len(vm.condStack) == 0 {
  		str := fmt.Sprintf("encountered opcode %s with no matching "+
  			"opcode to begin conditional execution", op.opcode.name)
  		return scriptError(ErrUnbalancedConditional, str)
  	}
  
  	conditionalIdx := len(vm.condStack) - 1
  	switch vm.condStack[conditionalIdx] {
  	case OpCondTrue:
  		vm.condStack[conditionalIdx] = OpCondFalse
  	case OpCondFalse:
  		vm.condStack[conditionalIdx] = OpCondTrue
  	case OpCondSkip:
  		// Value doesn't change in skip since it indicates this opcode
  		// is nested in a non-executed branch.
  	}
  	return nil
  }
docs: // opcodeElse inverts conditional execution for other half of if/else/endif.
  //
  // An error is returned if there has not already been a matching OP_IF.
  //
  // Conditional stack transformation: [... OpCondValue] -> [... !OpCondValue]