A plan step Step represents an action. It may have enabling preconditions. It has at least one effect Eff. It is the instantiation of an operator.
A plan step may be a decision-step Decide. A
decision-step has enabling preconditions of the form
(know-if Cond) for a condition Cond. Decide also has
a set of decision-rules.
An effect Eff represents some results of an action. It is attached to a step Step, representing that action. It may have secondary preconditions. It has at least one postcondition Cond, a condition that becomes true as the result of executing Step when the secondary preconditions hold.