Sentence:  Not long ago, it was difficult to produce photographs of tiny creatures with every part in focus....A photo processing technique called focus <WSD>stacking</WSD> has changed that. Developed as a tool to electronically combine the sharpest bits of multiple digital images, focus <WSD>stacking</WSD> is a boon to biologists seeking full focus on a micron scale. 
Meaning of the word marked with <WSD> tags:  To arrange in a stack, or to add to an existing stack.