Undocumented predicates

The following predicates are exported, but not or incorrectly documented.

 unify(Arg1, Arg2)

 find_key(Arg1, Arg2, Arg3)

 role(Arg1, Arg2, Arg3)

 close_btree(Arg1)

 distribute_constr(Arg1, Arg2)

 region_constr_of_leaves(Arg1, Arg2, Arg3, Arg4)