Wrong definition of fo:line-height ?

Hello

The current definition of fo:line-height, like many other definitions, lacks 
many explanation about valid ranges of values.
But the fo:line-height definition allow a line-height of zero, which is wrong 
(or should be specifically described in the specification to make an 
implementation possible). The schema should not use nonNegativeLenght but 
positiveLength instead, to explicitely exclude 0 values.
Maybe a positivePercent should be defined too for such attributes, since the 
percent value allows even negative percent that are likely invalid too...

Thanks
  Pierre