Textual specification about unary devide

[dalito]

All formal grammar descriptions (e.g. Syntax in BNF form in the specification) allow a unary divide operator only in the front but not later. So m.(/s) is for example invalid. However, the specification text still says: The division operator can be used as a binary and unary operator, i.e. a leading solidus will invert the unit that directly follows it. - This should be reworded to match the formal definitions.

Where unary divide is allowed, was made clear in this commit in 2013 which extended the BNF syntax (see file ucum-source.xml)