Logtk.Test_propSourceTest universal properties that use defined functions, within some "reasonable" bound (e.g. smallcheck/quickcheck).
This is used to test a property semantically (by evaluation) before trying the costly proof by induction; if the property is false there is no need to try to prove it.
check_form rules form returns R_ok if the property seems to hold up to depth, or R_fail subst if subst makes form evaluate to false