The solidity docs say this about mappings, but don't give a reason why:
Mappings can only have a data location of storage and thus are allowed for state variables, as storage reference types in functions, or as parameters for library functions. They cannot be used as parameters or return parameters of contract functions that are publicly visible. These restrictions are also true for arrays and structs that contain mappings.
Why can they not be used as params or return values of public functions?
