covering index
      
When two range predicates were combined under an OR
predicate, the algorithm tried to merge overlapping ranges
into one. But the case when a range overlapped several other
ranges was not handled. This lead to

1) ranges overlapping, which gave repeated results and 
2) a range that overlapped several other ranges was cut off.  

Fixed by 

1) Making sure that a range got an upper bound equal to the
next range with a greater minimum.
2) Removing a continue statement