Thankyou all! The problem was solved. Dan Rathbun Do you mean the whole class "Exportvertices2csv" should be a module? Or, the instance method "printcsv"should be a module?

Thank you for all your imformation! UML seems to be not so popular. But thank you for the link!!

Dan Rathbun Thank you so much for instruction. I am now studying "Design Patterns in Ruby" And just learned about Singlton Pattern. Does every one use Design Pattern making sketchup plugin? Or maybe use UML?