Made the structure library compilable in void-safe mode with ISE 7.3.
Fixed bug in feature `search_position'.
Fixed bugs in DS_BINARY_SEARCH_TREE_SET.
Fixed bug: months should be added before days.
Take care of storable mismatch.
Got rid of the dummy item at index 0 in arrayed structures, which is incompatible with void-safety.
Added iteration routines to the descendants of DS_TABLE so that the key can be passed first and then the item to the agents.
Workaround a precondition violation deep inside the implementation of DIRECTORY.