Variable Traffic Sign: VARIABILITY_VARIABLE

[jdsika]

In v3.6 we further detailed the definition of the ENUM VARIABILITY in #683 . As a result we kept the definition:

 // Sign that can change semantically and/or positionally, i.e. it is
 // left unspecified in which way the sign may change.
 //
 // \note This value represents a legacy definition. If possible, please
 // use the more specific values below to indicate the exact nature of
 // variability.
 //
 VARIABILITY_VARIABLE = 3;

It is to be discussed if this is already covered by VARIABILITY_OTHER and if the value above can be omitted in a major version change.