3-d references

Hi,

Reading through section 5.8, I have a question about 3-d referencing.  Given a reference of the form:  =SUM(['Sheet1'.B4:'S
heet2'.C5])

Does the specfication explicitly define how to interpret references of this type when there are more than 2 sheets involved?  Excel, for example will sum the cells in the specified workbooks AND all cells in sheets that are positioned between the 2 sheets.