Fix problem on Ubuntu and NetBSD Builds
Both Ubuntu and NetBSD builds fail because of what appears to be an error trying to the use "equal_range" method of one of the classes associated with the Boost multi-index container. The cause is uncertain - other systems build successfully, so the problem might no lie here, e.g. it could be in the compiler. This fix attempts to side-step the problem by modifying the affected files so as not to use this method.