The Undefined class implements the value used in uninitialized variables and array members. There is one distinguished instance of the Undefined class in the reserved system variable undefined.
-- check to see if MyLight has a target
if MyLight.target != undefined then
-- distance from light to target
dist=distance MyLight.pos MyLight.target.pos
-- distance from light to [0,0,0]
-- set CH_Hand1 to undefined so can test later to see if deleted
deleteChangeHandler CH_Hand1; CH_Hand1=undefined