The dontCollect value is used in for loops to signal that the value is not to be collected. See For Loop for details. dontCollect is a separate distinguished instance of the Undefined class, and so will fail in the way the undefined value does if used in computations.
-- collect all spheres with radius less than 5
for i in $* collect
if classOf i == sphere and i.radius < 5 then i else dontCollect