What I find usually works is working back from the proposed implementation to the goal that it is supposed to serve. You can usually do this in a validating rather than rejecting way. Once you've identified and validated the goal you can address cover a with the proposed approach and present alternatives without the proposer viewing it as a rejection since you got to common ground on what you are trying to achieve before proposing alternatives.

Often the problem with an initial "no" to a proposal is that the proposer identifies the proposal tightly with the purpose and suggesting an alternative to the proposal without validating the goal reads as rejection of the goal.