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    <div id="details">
    <h2>Details</h2>
    <p>Use a one-sided formula to compute summary statistics for the right hand side
expression over the entire data.
Use a two-sided formula to compute summary statistics for the left hand (response)
expression(s) for each combination of levels of the expressions occurring on the
right hand side.
This is most useful when the left hand side is quantitative and each expression
on the right hand side has relatively few unique values.  A function like
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