@Override
    protected Map<String, List<Object>> mergePersonAttributes(final Map<String, List<Object>> toModify, final Map<String, List<Object>> toConsider) {
        for (val sourceEntry : toConsider.entrySet()) {
            val sourceKey = sourceEntry.getKey();
            if (!toModify.containsKey(sourceKey)) {
                val sourceValue = sourceEntry.getValue();
                toModify.put(sourceKey, sourceValue);
            }
        }
        return toModify;
    }
Merger which implements accumulation of Map entries such that entries once
established are individually immutable.

@author andrew.petro@yale.edu
@since 7.1.0
core/cas-server-core-authentication-api/src/main/java/org/apereo/cas/authentication/principal/merger/NoncollidingAttributeAdder.java