An example of a compound with a Dawson lacunary structure is AsWO. The species [PWO] is a lacunary, or defective Keggin ion. The final part of the text has been transmitted in a lacunary form. Such a power series is called lacunary. When an exception exists, it is called a lacunary value of the function. The word "lacunary" is derived from lacuna ("pl." lacunae), meaning gap, or vacancy.