interval
since we are building interval trees, have element "interval" for consistency* with avltree and splaytree (*actually use of a pointer is inconsistent)
Data payload / encapsulated object ( modified also in cgranges.h/.c by JSB)
NOT used
prior to cr_index(), x = ctg_id<<32|start_pos; after: x = start_pos<<32|end_pos
See Implementation
interval