flip = function(favor) {
    return isLeftHandSide ? favor :
        (favor == Favor.LHS) ? Favor.RHS : Favor.LHS;
  }
Directionally compare two keys.
Left hand side key comparison logic: null is considered left, exclusion
is considered right.
Right hand side key comparison logic: null is considered right, exclusion
is considered left.
@param {?lf.index.Index.SingleKey|!lf.index.UnboundKey} l
@param {?lf.index.Index.SingleKey|!lf.index.UnboundKey} r
@param {boolean} isLeftHandSide
@param {boolean=} opt_excludeL
@param {boolean=} opt_excludeR
@return {!lf.index.Favor}
@private