The Unsupplied class implements the value used to initialize unsupplied keyword parameters to functions. There is one distinguished instance of the Unsupplied class in the reserved system variable unsupplied. You can test to see whether a caller has supplied an optional keyword argument by comparing it with unsupplied.
fn walk_tree obj parent: =
if parent == unsupplied then
FAQ: What is the difference between 'undefined' and 'unsupplied' values?