+ $from_terms = common_license_terms($from);
+ // public domain and cc-by are compatible with everything
+ if(count($from_terms) == 1 && ($from_terms[0] == 'publicdomain' || $from_terms[0] == 'by')) {
+ return true;
+ }
+ $to_terms = common_license_terms($to);
+ // sa is compatible across versions. IANAL
+ if(in_array('sa',$from_terms) || in_array('sa',$to_terms)) {
+ return count(array_diff($from_terms, $to_terms)) == 0;
+ }