simple dependency intends this behaviour:

	component status		effect
expression	component a	component/feature b
a (:depends-on (:b))	present	present	(operate b), (operate a)
a (:depends-on (:b))	present	absent	error
a (:depends-on (:b))	absent	present	error
a (:depends-on (:b))	absent	absent	error

given the code in parse-component-form, :weakly-depends-on would appear to effect this behaviour:

	component status		effect
expression	component a	component/feature b
a (:weakly-depends-on (:b))	present	present	(operate b), (operate a)
a (:weakly-depends-on (:b))	present	absent	(operate a)
a (:weakly-depends-on (:b))	absent	present	error
a (:weakly-depends-on (:b))	absent	absent	error

there are cases, where it is useful to cause a third behaviour:

	component status		effect
expression	component a	component/feature b
a (:contingent-on (:b))	present	present	(operate b), (operate a)
a (:contingent-on (:b))	present	absent
a (:contingent-on (:b))	absent	present	error
a (:contingent-on (:b))	absent	absent	error

perhaps there are others. this one is useful when building a system where the components are contingent on the runtime and/or o/s.