Functionality for manipulating values of associative maps. The package is designed to be used with the 'mvp' class of packages that use the STL map class: its purpose is to trap plausible idiom that is ill-defined (implementation-specific) and return an informative error, rather than returning a possibly incorrect result. To cite the package in publications please use Hankin (2022) <doi:10.48550/ARXIV.2210.03856>.