OData-CSDL, section 14.4.7 allows If without third ("else") child only as a direct descendant of a Collection. But the following would also make sense:
<Collection> <If> <Path>IsLTR</Path> <String>margin-left</String> <If> <Path>IsRTL</Path> <String>margin-right</String> </If> </If> </Collection>
This would add "margin-left" or "margin-right" to the collection for left-to-right and right-to-left scripts, respectively, but add nothing, e.g., for top-to-bottom scripts. The If without "else" is embedded in another If.