Under 5.17.2 Ceiling, we issue a warning?
"Warning: Many application user interfaces have a CEILING function with only two parameters, and somewhat different semantics than given here (e.g., they operate as if there was a non-zero mode value). These CEILING functions are inconsistent with the standard mathematical definition of CEILING. Such applications shall convert such formulas into a 3-parameter format when saving in OpenFormula format, so that the saved formulas work correctly."
Err, perhaps a note?
But we can't constrain the behavior of such applications. We can specify OpenFormula format but not the behavior of foreign applications.