lambdacd.step-id
child-id
(child-id parent-step-id child-number)
direct-parent-of?
(direct-parent-of? parent child)
later-than?
(later-than? a b)
parent-of?
(parent-of? parent child)
root-step-id-of
(root-step-id-of step-id)
root-step-id?
(root-step-id? step-id)