Draw the main chain minus 2 carbons, and add two one-carbon groups (two methyls) or one 2-carbon group (an ethyl) to as many positions possible, trying not to repeat structures. Continue subtracting and adding groups in this fashion until you run out of carbons or doing so only results in repeated structures. Apr 24, 2007 · Most steps in a process map will state that some work is done in that step, before the process flow continues. Often this work involves use of a system, which can be documented as a use case. The use case then provides context for declarative requirement statements, as well as a cross-reference to data items and their definitions in a data model.