This update for units fixes the following issues:

units 2.21:

  * Fix bug arising when some environment variables are undefined
  * Improve error reporting when input files are missing or invalid

units 2.20:

  * Add abs(), ceil(), round(), floor(), Gamma(), lnGamma(),
    factorial(), erf() and erfc() functions.  
  * Add densities of all of the elements  
  * Add several new unit systems
  * Add --conformable option which noninteractively prints a list
    of all units conformable with a specified a specified unit
  * Invoke the pager more intelligently depending on the size of the
    screen
  * Fix bug where unit lists conversions displayed excess precision
    (e.g. non-integer output for integer input)
  * Display precision warning when conversion to unit lists uses
    all of the available precision

units 2.19:

  * Unit definitions updated to reflect new 2019 revisions to SI
    and the 2018 NIST CODATA.
  * Added definitions of country and USA state areas, and physical
    properties of selected woods.
  * Changes only to definitions.units in this release.  

units 2.18:

  * The units_cur script now supports multiple currency sources
  * Add hyperbolic functions: sinh, cosh, tanh, asinh, acosh, atanh
  * Add log to arbitrary integer bases: log3, log4, log72, etc.
  * Units can be redefined without a warning by adding '+' before
    the unit name in definitions.units.  
  * Add 'quit' and 'exit' commands.
  * Add support for CGS electromagnetic units (activated using the
    -u option or the UNITS_SYSTEM environment variable)
  * Add !prompt command to add a prefix to the "You have:" prompt
  * Texts specified with !message appear in the log file

units 2.17:

  * Updates to units database
  * Update units_cur to handle bad records returned by yahoo server
  * Move currency.units out of /usr/share because the file is not
    static and /usr/share is supposed to house static files. 
    The new location is $sharedstatedir which may be /var/lib