Clojurians Log v2

Given an incomplete map and its spec, can I get spec to give me back the list of missing keys (required and optional)? With explain-data I can get the required keys but what about optional ones? Context: I'm playing with the idea of (dynamic) spec-driven config presets, i.e. there's a process that creates named incomplete maps and then another process/user selects a preset and fills-in the gaps.