concat expresion

`opinputpath("..", ch("spare_input0"))`

../../`opinput("..", 0)`/`$OS`

https://sites.google.com/site/yuricgvfxmemo/home/houdini/expressions

http://lex.ikoon.cz/op-expressions/

http://nomoreretake.net/2016/03/07/houdiniopinput/