is {} a union of elements of {{a,b},{b},{b,c}}?
is {a,b} a union of elements of {{a,b},{b},{b,c}}?
is {b} a union of elements of {{a,b},{b},{b,c}}?
is {b,c} a union of elements of {{a,b},{b},{b,c}}?
is {a,b,c} a union of elements of {{a,b},{b},{b,c}}?