search for: unabletolegalize

...erges that we can handle, then this query would return false and the combine would not work. But we also cannot create a rule for those unmerges without causing endless loops through legalization. The way we get around this problem is to add a custom rule, in which we simply return false (i.e. UnableToLegalize). This is enough for the combiner to produce the intermediary unmerge (because it is now support through the custom rule), while also getting around the problem of expanding something we don't want in the legalizer (because we say we didn't legalize anything). This is obviously a hack....