Usage: unk CALCULUS

* Valid CALCULUS are:
        1993/12/24 + 93
        today - Mar 8, 1965
        12 [== today + 12]
        3/8/65 [== today - Mar 8, 1965]
        3 8 1965 + 76.245 [year, regarded as 365 days]
        ...
* Numerical date is interprited as: 3/8/93 (Mar. 8th, American)
  => 93/3/8 (Mar. 8th, Japan) => 24/12/83 (Dec. 24th, English).
* `@' `today' are substituted by today's date
  `!' `last' are substituted by a result of the last calculus
* Special commands are:
    `help' `?': print this message
    `gpl'     : print GNU General Public License
    `exit'    : end of interaction