BibTool 2.68

• Handling of extracting by aux file improved.
• Record extended by attribute lineno.
  This attribute carries the line number of the initiating @.
• The format of the error messages has been slightly streamlined.
  Double spaces are avoided; two colons in one message are avoided.
• The format of the messages of check_rule include file and line
  number.
• Warning for double fields added.
• The empty output file is used to signal that the output should be
  suppressed.
• The resources check.warning.rule and check.error.rule
  have been introduced to allow semantic checks to be classified as
  warning or error.
• The behaviour of the resource check.double has been
  generalized. The requirement that double entries to be adjacent
  has been dropped. This has the impact that the processing is
  slightly slower.
• New resource file unique.field introduced. With this
  resource it is possible to specify additional unique constraints
  for fields. If different records have the same value for one of
  those fields then a warning is issued.
• Static library renamed from libbib.a to libbibtool.a.