yang-revisions.txt

general: get rid of repetition xxx-arg & xxx
identifier:  non-quoted
module: identifier
prefix: identifier (error if have colon)
revision: should be just an iso-date. Ex:  2009-10-23
string: plain double-quotes, no collapsing
typedef: name -> identifier;
  type:  name -> identifier;
  length:  string
  reference:  string