I am designing a graph algorithm. Some steps of the algorithm, are set operations (union, difference, intersection, set-membership).Can I assume them as $~ \mathcal{O}(1)$ operations? Have someone used them as $~\mathcal{O}(1)$ in his paper?What minimum complexity should I assume for these operations? References are welcome.  
Complexity of set oprations in algorithm
algorithms;data structures;sets