In some cases, it is desirable to allow an equipment class to be specified with varying degrees of detail. For example, you may not currently know the height of the skirt for a vertical vessel, but will later. To accommodate both options, the Vertical Vessel class has a skirt_height attribute so that you can input a value. The length clause for the skirt, however, uses the logical operator “OR” that provides for a default height should the skirt_height attribute is not provided:

length =	equipment.skirt_height or
	equipment.length / 4.0