Require a minimum fanout for interior nodes. This prevents cases

Require a minimum fanout for interior nodes.  This prevents cases
where excessively large terms keep the tree from finding a single
root.  A downside is that this could result in large interior nodes in
the presence of large terms, which may be prone to fragmentation,
though if the nodes were smaller that would translate into more levels
in the tree, which would also have that problem. (CVS 3510)

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