How to set discountOverlaps="true" in Solr 4x schema.xml

[Tom Burton-West]

If I am using solr.SchemaSimilarityFactory to allow different similarities
for different fields, do I set "discountOverlaps="true" on the factory or
per field?

What is the syntax?   The below does not seem to work

<similarity class="solr.BM25SimilarityFactory" discountOverlaps="true"  >
<similarity class="solr.SchemaSimilarityFactory" discountOverlaps="true"
 />

Tom